Dungeons & Dragons - A Guide to Devils

I might be just about done with this! I'll probably touch it up as I work on the Guide to Hell.

This guide is a broad overview of devils in Dungeons & Dragons. "Devil" is a term that encompasses a lot of different creatures that are organized into a hierarchy.

First, we'll go over 5th edition devils. After that, we'll look at general devil traits and weird little things from other editions that might add to your game. Once that is done, there is a list of every devil type that I can find.

Purpose: The idea here is to mine as much material as possible so that you can use them as fodder for your campaign.

I used a ton of sources on this. I think that if you are looking for a supplement to give yourself more devil-related things to use in your campaign, I suggest you go with one of these:

• Fiendish Codex II
• Guide to Hell
• Faces of the Fiends

Faces of the fiends is very overlooked, I think, because of the name. Whenever I see the title I immediately think it is a book full of NPCs. It's not. It is a treatise on devils, demons and yugoloths. It is really good.

Devils and Demons: It is important to understand that devils and demons are two entirely different types of creatures. They are enemies. It goes like this:

• Devils are lawful and they live in Hell.
• Demons are chaotic and they live in the Abyss.

It is Chaos vs. Law. The eternal war between these fiends is known as the Blood War.

Devils in 5th Edition D&D

Can a Devil Die? If a devil dies outside of the Nine Hells, it instantly returns to its home layer, reforming at full strength. Devils only die if they are killed in the Nine Hells.

What Devils Do: Here's how devils think and operate:

• They conquer, enslave and oppress.
• They obey orders even when they dislike their superiors.
• They love to strike bargains with mortals in exchange for their soul.
• "Only divine intervention can release a soul after a devil has claimed it."

What is a Baatezu? Baatezu is another name for devil. "Baator" is another name for the Nine Hells. In 2nd edition, devils became known as "baatezu" due to pressure from groups that thought that D&D was tied to satanism and the occult. They didn't even call Asmodeus by name until the very end day of 2nd edition.

The 5e Devil Hierarchy

This 2nd edition diagram shows possible promotions

Most devils in 5th edition have these traits:

• Resistant to cold and nonmagic weapons that are not silvered.
• Immune to fire and poison.
• Magic Resistance: Advantage on saves vs. spells.
• Devil's Sight: Magical darkness doesn't impede their darkvision.
• Telepathy 120 feet.
• They speak and read the Infernal language.

Pit Fiend

Because devils are all about order, they have a strict caste system. Devils can be "promoted" or "demoted" from one type to another. They're all gunning to become an archdevil who rules one of the nine layers of Hell.

Here's the 5e version of the hierarchy:

1. Lemure: The lowest form of devil. Can only be truly killed with a blessed weapon or holy water. Can't speak, but babbles.

Lesser Devils
2. Imp: Spies, willing servants of mortals.
3. Spined Devil: Messengers and spies for greater devils and archdevils, flying artillery.
4. Bearded Devil: Violent shock troops.
5. Barbed Devil: Guards who are very alert.
6. Chain Devil: Sadistic jailers and torturers.
7. Bone Devil: Cruel taskmaster of devils beneath them.

Greater Devils
8. Horned Devil: Lazy and belligerent flying infantry.
9. Erinyes: Beautiful, fierce and disciplined winged warriors.
10. Ice Devil: Commanders of the armies of the nine hells.
11. Pit Fiend: Lords of other devils, generals of armies,  direct servants of Archdukes.

Archdevils
12. Duke or Duchess: Unique devils that serve or scheme against archdevils.
13. Archduke and Archdutchess: Rules one of the nine layers of Hell.

The Rulers of Hell

Mephistopheles, Lord of Cainia

While I wrote this guide, I also wrote guides to Hell and the archdevils. Here is a quick overview of both:

There are 9 layers of Hell, each with its own ruler:

1. Avernus, ruled by Zariel
2. Dis, ruled by Dispater
3. Minauros, ruled by Mammon
4. Phlegethos, ruled by Belial and Fierna
5. Stygia, ruled by Levistus
6. Malbolge, ruled by Glasya
7. Maladomini, ruled by Baalzebul
8. Cania, ruled by Mephistopheles
9. Nessus, ruled by Asmodeus

Asmodeus is the ruler of all devils.

Tiamat: In 5th edition, Tiamat the evil dragon god is trapped in Hell (I wrote a Guide to Tiamat here). The adventure Rise of Tiamat is all about the Cult of the Dragon performing a ritual to free Tiamat and bring her to the Forgotten Realms.

1. In 1st edition, Tiamat actually ruled Avernus, the first layer of Hell.
2. In 2nd edition, the heroes can actually go to her lair in Hell in the adventure Fires of Dis. 
3. In 3rd edition, Tiamat has a massive army called the Red Hand of Doom that tries to overtake the Elsir Vale (a generic fantasy realm). 
4. In 4th edition, she lives in the astral plane (called the "astral sea" in 4e) on a floating island called Tytherion. There is a huge adventure path called Scales of War which involves Tiamat killing Bahamut, the god of good dragons. In the end of the whole thing, the group can actually kill her, which may have sent her back to Hell where we find her in 5th edition.

Things to Know About Devils

A narzugon and a spinagon

There are tons of great ideas from older editions that you might want to mine for your game.

The Pact Primeval: Described in the Fiendish Codex 2, this is an agreement signed by lawful deities that established the system of punishing the damned. Asmodeus exploits the loopholes and uses them to his advantage. There are 3 copies of this contract. There is one in Hell, one in Mechanus and one in Mount Celestia. These contracts are heavily protected and radiate immense power.

Language of the Devils: Devils speak and write a language called Infernal. It is a rigorous language with rigid grammar. In Infernal, there is only one way to construct any given statement.

The alphabet uses 33 geometric glyphs. It's a good language for book keeping but it is bad for poetry. There are four forms of Infernal. Speaking a higher tongue of Infernal than permitted will result in the offending devil being punished for insolence. Each form is more complex than the last:

• Least Infernal: This is very basic and is used for simple commands and insults.
• Lesser Infernal: This is the form most humanoids learn. Those who peak this can understand the higher forms, but aren't fluent in them.
• Greater Infernal: This showy version has patterns that emerge over the course of a conversation.
• Mabrahoring (High Infernal): This is an archaic form of the language spoken mostly by archdevils. Only unique devils can learn and speak this language. Comprehend languages can decipher it.

Promotion: Archdevils can transform a devil into a higher form. This is often done as a reward for service. The promoted devil keeps their memories from their previous form. Only 1 in 1,000 lemures get promoted.

Being Promoted: When a devil is selected to be promoted, they go through a painful ceremony that lasts up to a day. Their old form breaks apart and the new form emerges like a parasite via the ovatorium (described in "Biology" below). They retain full memories of their devil history.

Promotion Expunges Chaos: Devils purge themselves of chaotic behavior and thoughts as they get promoted up the ranks. The highest-ranking devils, ice devils and pit fiends, are thoroughly lawful and rigid.

Pain and Promotion: Being promoted is not fun. It is said that the faster you got promoted, the more painful your transformation. Every time a devil achieves a new rank, they experience a different kind of pain.

Here are some examples:

• Thrown into the pit of flame where your impurities are burned away.
• Your skin/hide is shorn off, revealing your new form underneath.
• Masters of torture cut and reshape your body into the new form.
• Spined devils go through an undefined process called the Ritual of Spined Descent.

The Ministry of Promotion: This ministry is overseen by Zaebo of the Dark Eight. They record and scrutinize each promotion. There are stringent requirements to be met, and superiors making bad promotions are punished by being placed on the front lines in the blood war.

Things to know:

• None may rise until another falls - a spot must open up in order to be promoted.
• Generally, incompetents don't rise in power. Backstabbers do. 
• Speedy promotions are discouraged.
• Devils don't reward the good - they punish the bad. Exemplary service means no mistakes.
• There is a lot of intrigue. Lots of doctoring reports to keep a certain devil down.

Demotion: The punishment for failure or disobedience is demotion. If demoted to a lemure, the devil loses memories of their former status.

Barbed Devil (Hamatula)

Where Did Devils Come From? Each edition of D&D changes the story a little. To me, the most interesting one is from 2nd edition. That one claims that there were creatures in Hell before the devils. These creatures "disappeared."

It goes like this. Soul larvae of Hell are evil souls not touched by devils in life. If left alone, they become nupperibos, not lemures. If the nupperibos are left alone, they will become.. something else. Apparently they'd become creatures called Ancient Baatorians, eventually. Nupperibos and amnizu are the only two types of devils who actually get promoted 'downward.' That's because they're not being promoted - they're being transformed from Baatorian to Devil.

There are some clues that the Ancient Baatorians are deep underground and that they may have created the devil race. In Faces of the Fiends it is shown that devil skeletons are clearly made rather than grown naturally.

Devils are Ambitious: All devils are ambitious. They hate defeat and humiliation. They pursue vengeance with tremendous vigor.

Devils Don't Have to Eat: Devils derive sustenance from the energy of tortured souls. They can eat if they want. They enjoy the flesh of angels, demons, and whole soul larvae.

Eating Lesser Devils: If you eat a nupperibo or lemure, they are gone forever. If their essence is ingested by another, it is erased from existence. This is why demons devour them on the battlefield.

Devils Need Oxygen: Devils do need oxygen (according to the fiendish codex II). Not much, but some.

Reproduction: Every edition says something different. 2nd edition says that males are fertile, but females are not. 3rd Edition directly contradicts this, saying that only an erinyes can become pregnant. The more humanlike a devil is, the stronger the desire for sexual gratification.

Tieflings: Tieflings do not have a devil parent. Tieflings are descendants of devils, never more than 1/4th devil.

Pit Fiend and Ice Devil

Intoxication and Alcohol: Alcohol doesn't intoxicate them. In Hell, devils drink:

• Gughalaki: A sweet, ropy, hallucinogenic liquid made from fiendish centipede glands. 
• Infernal Wine: Distilled from the fire grapes of Phlegethos.
• Screecher: A dulling acrid tipple.

Biology: Inside the bodies of devils are organs like the ones humans have, plus more. Their bodies contain the ovatorium - mushy tissue littered with dozens of tiny sacs. In each sac is finger-sized fetal versions of different devil types. When promoted, one of those little devil fetuses grows, explodes out of the body and becomes the new form of the devil.

Here are other anatomical differences:

• The adrenal gland is three times the size of a human adrenal gland, making devils aggressive and quick.
• Their warped muscles give them fantastic endurance.
• Their blood is black, but it changes colors in different atmospheres.
• Their interior organs are covered with scales - natural armor
• Their bones are slightly metallic and seem like they've been carved/constructed.

Erinyes with an infernal contract

Gender: 1/3rd of the devil population is male, 1/3rd is female, and 1/3rd are genderless. Fun facts about devil gender:

• Lemures and nupperibos are genderless. 
• Erinyes are always female but can assume a male form.
• Pit fiends get to pick their gender, if any. They can even change their mind and switch, but they will be punished for disturbing the order. They must spend three days in the pit of flame if they change their gender.

Getting Old: Devils don't physically age. That said, in 1st edition some devils are specifically described a being middle-aged and have receding hairlines.

Sleep: Some books say that devils don't sleep, some say they do. Those that say that devils sleep explain that they sleep one hour out of every nine. Devils have violent, evil dreams that reinforce lawful behavior. They may join in a devilish collective unconscious when slumbering.

Devils Dying: There is a specific name for when you die and re utterly obliterated, unable to come back in any way: True Death. Devils fear this more than anything. Different editions handle this differently. In every edition, a devil experiences True Death when it is slain in Hell.

When slain somewhere else, the devil appears back in hell. In some editions, the slain devil appears in Hell instantaneously. In others, they reform over the course of 99 years.

1st edition has specific rules on this:

• If a greater devil has its material form destroyed, it becomes a lemure, is tortured for 9 years and then it is returned to its former station. 
• If a duke is destroyed, it is bound to hell for 10 years. 

In 2nd edition: 

• If a devil chooses of their own free will to leave Hell and dies, it experiences True Death. 
• When slain outside of Hell, lesser devils are reborn as nupperibos. They do not retain their memories, but regain them when promoted above lemure status.
• Almost every devil has a recovery plan in case they end up as a nupperibo due to being slain. A lower-ranking lieutenant has instructions to find them and promote them.

Your Choice: Deciding what happens when a devil is slain makes a huge difference, so you should definitely think carefully about it. This is an easy way to get around the problem of using a recurring villain who always escapes in some contrived manner.

Devils With Special Powers: Some devils were part of a failed experiment that gave them one of two abilities. When they use a power, they "drain their life essence." These devils look utterly emaciated:

• Illusionist Devils: Some devils can cast illusion spells at the cost of draining their life essence.
• Blind Devils: They can fire magic missiles from their eyes every round, but they take the damage, too. A lesser fiend has to guide them about, using telepathy to direct their attacks. If a blind devil is promoted, it is no longer blind and can no longer fire magic missiles.

Devil True Names: Every devil above a lemure has a true name. If you learn it, you can summon that devil and bind them into service. Usually this requires a ritual/sacrifice and possibly a devil talisman.

Devil Talisman: An ancient relic that has the truename of a devil on it. Usually, it was bathed in the blood of someone the creator loved when it was crafted. 1st edition has specific rules about how a talisman affects different ranks of devil:

• You can use a devil talisman to force a devil to serve you for nine days.
• Greater devils serve for nine hours
• Archdevils will do one single service for you.

A 3e erinyes being summoned

Economy: Money doesn't do much for devils. They trade for gems, soul larvae, knowledge, magic and favors.

Devils Want Souls: Devils want souls. They don't want to obliterate them, they want to torture them. The torment of souls nourishes devils. The more tortured souls an archdevil has, the more powerful the archdevil is. Devils lure souls to Hell by corrupting mortals either through guile or through a devil pact, described below.

Some Souls Are More Valuable: In the 4e Book of Vile Darkness, it says: "The brighter a soul shines, the more energy the Hells can squeeze from it." So, the more powerful or good the soul is, the more energy devils can draw from it.

Devil Law: Legally, a superior devil can always attack a lesser-ranked devil. Devils can do anything violent to other devils that are nine stations lower or more. Any other assault is illegal, as injuring a devil is considered to be an act of "damaging the property" of an archduke.

Devils can file for a license of lawful combat, where they can fight in the duelist's chasm in Stygia.

The Ministry of Mortal Relations: They govern all contact with mortals. Devils who are summoned by a mortal spellcaster must report to them when it is over. The Ministry will want all information that the devil gleaned while in captivity.

Devil Pacts: In the Fiendish Codex, there are two types of devil pacts:

• The Pact Certain: This is an explicit contract where a mortal affirms their allegiance to a lord of hell in exchange for whatever benefits are offered. This can be voided if the signature is given under duress. It must be given freely. Those who sign one a contract to free a soul from Hell do not count as being under duress.
• The Pact Insidious: This delivers its promises in stages. The signer will need to do a series of tasks, each nudging them closer to damnation.

The other way to get out of a pact is to prove that the devil did not provide the benefits promised.

I would very highly recommended checking out the way that pathfinder does infernal contracts (scroll down a bit on this page here). I think they are really good.

Where Devils Come From: When you die and go to Hell you appear on the Shelves of Despond, clammy rocks on a half-mile path of the River Styx. If a devil corrupted you, you bear the brand of an archdevil on your soul.

In general, almost everyone appearing in hell has a brand that indicates that they are the property of one of the nine archdevil rulers. Bearded devils scour the river for these soul shells, haul them into cages and transport them to a torture station.

The torture stations are run by amnizu. The amnizu record the new arrivals in meticulous logbooks and files. The soul shells are then stripped of their individuality through torture and their magical energy flows to the archduke of the layer.

The shells are thrown in a maggot pit and transform into lemures, the lowest form of devil.

No Memories of Mortal Life: Devils never retain the memories from their mortal lives.

Some Devils Become Good: Many devils are fallen angels of good. The reverse also occurs. Devils who turn good are known as The Risen. Hell's Ministry of Morale sends agents after them.

Types of Devils

You can go through this list of devils and pick out the ones that you would like to use in your campaign. You will see that many of the devils have two names, the second of which is in parenthesis. I put the more descriptive name first, as it is easier to remember.

Abishai

Abishai: These are draconic devils that appear in five different colors, each stronger than the last. They served Tiamat at one time. In the Fiendish Codex II, it says that some devils are promoted from lemure (the lowest form of devil) to abishai. Abishai progress from one color to the next. They start as white abishai, then black, green, blue and finally, red. Reds who are promoted become chain devils.

Advespa: These devils are in the 3rd edition Monster Manual 2. They are female, wasplike devils. Most of them are entirely black. They often serve amnizus.

Assassin Devil (Dogai)

Assassin Devil (Dogai): Assassin devils are gray-skinned fiends who can turn into shadows and summon fog to obscure their location. Outcasts among their own kind, assassin devils spy on and assassinate devils in the courts of pit fiends and archdevils. Many of them report directly to Asmodeus, feeding him the secrets of their patrons.

Ayeperabo Swarm

Ayperabos Swarm: This swarm of tiny flying devils can burrow into the flesh of a creature and take control of the creature's body. Devils and other fiends consider ayperabos to be a delicacy. Because of this, these swarms despise other devils and will attack them.

Barbed Devil (Hamatula)

Barbed Devil (Hamatula): These devils are guardians of places and individuals. They rarely leave Hell. In fact, they can not pass from one layer of Hell to another on their own. They have a gland behind their ears that produces a powerful hallucinogen. It is used to torment and interrogate prisoners.

Bearded Devil (Barbazu)

Bearded Devils (barbazu): These devils are shock troops of Hell. They often lead armies of lemures into battle. They love fighting, utilizing glaives and their disease-carrying beards (which sometimes are made of snake) as weapons. Most of them never survive long enough to get promoted to the status of bone devil.

Bone Devil (osyluth)

Bone Devil (osyluth): First off, the above drawing by Baxa is my favorite depiction of a bone devil by far. If you look at other artistic depictions, none really feels right except this one. Bone Devils are skeletal demons with scorpion stingers. Some of them wield hooked polearms that they can use to grab creatures with. They are the "police" of the Nine Hells, authorized to send anyone breaking the law into the pit of flame where they are tormented for 101 days.

There are 1,000 bone devils in existence. All of them are promoted to barbed devil every 100 years. A devil who kills a bone devil is demoted to a "marked" lemure. Marked lemures are never allowed to advance and they are hated by other devils.

• The Ring of Centrum: Once per century, 100 bone devils meet with the Dark Eight to choose which ice devil will be advanced to pit fiend status. 

Wizards of the Coast actually previewed the 5e bone devil as part of the Monster Manual release. You can see everything there is to know right here.

Brazen Devil

Brazen Devil: Encased in hellforged brass armor, brazen devils dwell in Nessus and serve as loyal guards. War devils get promoted to brazen devils, bone devils or barbed devils. They are cut apart and fitted into hellforged armor that can't be removed and will be attached to them for the rest of their existence. They wield hellfire halberds and can teleport.

Burning Devil

Burning Devil: These devils are made of flame and they despise the living. Mortals who live a life of extreme torment such as corrupt priests, necromancers and mass murderers often become burning devils.

There are two special types of burning devils:

• Blackfire Burning Devils: These devils burn with necrotic flames due to their dark mortal lives.
• Whitefire Burning Devils: Their fire is actually made of radiant energy, possibly because in their mortal life they were a holy servant.

Chain Devil (kyton)

Chain Devil (Kyton): Chain devils are literally covered in chains that sprout hooks and blades. They use their control of chains in their work as jailers and torturers. They primarily live in the city of Jangling Hiter on Malbolge, a city made entirely of chains.

Gorechain Devil

Gorechain Devil: I'm listing these as a variant of a chain devil because these creatures are basically ogre-sized kytons with chains that have a mind of their own. Gorechain devils are jailers and bounty hunters. They can use their chains to make a person their "puppet," controlling their movements in battle.

Hellchain Weaver

Hellchain Weaver: Kytons live in fear of these massive devil spiders made of hooked, jagged iron. They spin webs of iron chains and they have whirling blades embedded in their bodies. They regenerate their wounds and live in elaborate chain webs. They are speaking, intelligent creatures and they love to stalk and torment their prey.

Corruption Devil (Paeliryon)

Corruption Devil (Paeliryon): This hefty devil dons makeup and wears hellish perfume that fogs the mind of those nearby. They often have a network of informants who blackmail people into handing over their souls. These incredibly powerful devils serve Dispater and enforce his will in the City of Dis.

The Escapist declared this the 8th dumbest D&D monster of all time. I don't know if it's dumb or not, I think it depends on how you you run it and what your group is like. 4th edition added a lot of cool lore to them:

Sire of Corruption: The 4e Monster Manual 3 says that these creatures are more powerful versions of corruption devils. They go to mortal worlds to corrupt entire towns and kingdoms. They resent those who are highly moral and go out of their way to try to corrupt or kill them. Sires of Corruption can deal out a "touch of vice" that infuses a person with lust, excess, greed, fear, sloth, vengeance. This can turn them into a corrupted follower.

Corrupted Glutton

Corrupted Followers: These mortals signed a pact with a sire of corruption. The pact is actually burns red on their flesh. These creatures were often at a low point in life and handed over their soul in exchange for a need created in part by the sire of corruption's touch.

These creatures can be returned to normal, but the contract must be nulled and the corrupted must forsake their vices.

There are many types of corrupted followers:

Corrupted Lecher: These are extremely attractive people who others become infatuated with. People become so obsessed that they lose sleep over them. Corrupted lechers can cause lust with their touch and when they are in danger, can cause a "spread of passion" to try to influence the attacker to spare them.

Corrupted Glutton: These people become gluttons, devouring anything they like. They are minions of Sires of Corruption.

Corrupted Monger

Corrupted Monger: These people are consumed with the desire for wealth. They spread greed with their touch and have the ability to drain magic items of their power.

Corrupted Idler: Idlers are those dependent on others to provide for their needs. They want to live a life where everything is done for them. They cause sloth with their touch and have an aura that makes people near them weary.

Corrupted Lunatic: These mortals can't let go of their anger. They hold grudges and eventually become violent.Their touch causes rage and they have a number of powers that force people to attack others.

Corrupted Craven

Corrupted Craven: Soldiers who were overcome with fear and abandoned their posts, leaving the people they were supposed to protect to a grim fate. They have the power to infuse people with fear and give them the frightened condition.

Erinyes

Erinyes: Erinyes change with each edition. I think this is partly because they were too similar to the succubus once the succubi became devils. In 2nd edition, they were tempters of mortals who reported directly to the Dark Eight (the pit fiend council that governs much of Hell).

It is said that there are only 500 erinyes in the Nine Hells at one time, but if you read enough products it seems like there is a heck of a lot more than 500 in existence. They have the ability to pass into the prime material for a short time. They can only bring one person back and no inorganic matter.

When a mortal is lured to hell by an erinyes and dies, it becomes a lemure. Many erinyes refuse promotions - they don't want to go up in rank. Some erinyes are directly promoted to pit fiend. Most become ice devils. Some become pleasure devils.

3rd edition bearded devil, war devil and erinyes

In 3rd edition, the erinyes are rumored to be angels who fell. They are depicted as having blue skin and red hair. They are scouts, servants and concubines. It is said that they are fallen angels who fought alongside Asmodeus when he rebelled long ago, and they fell into Hell with him.

Erinyes often see themselves as superior to other devils. Other devils feel animosity toward them, but also envy, lust after and idolize them.

In 4th edition, Erinyes became wingless monstrous females who were elite warriors. "Furies of Vengeance and rage, erinyes exist for battle." They have attacks that give allies temporary hit points.

In 5th edition, they are still elite warriors but they are beautiful females and males often mistaken for celestials.

Harvester Devil (Falxugon)

Harvester Devil (Falxugon): These creatures go to mortal realms to corrupt as many souls as they can. Harvester devils have an aura that makes it hard for mortals to attack them. They need to make a saving throw or else they can't bring themselves to do it.

Hellcat (Bezekira): In 3rd edition, Hellcats were listed as stronger than chain devils and weaker than an erinyes. They were featured as one of the main devil types. They are telepathic cats and lions made of blinding light and fiery sparks.

Hell Hound

Hell Hound: Hell hounds are often found serving devils and fire giants. They can breathe fire and are thoroughly evil. They care most about eating flesh, as it stokes the fires within them.

Hell Knight (Narzugon)

Hell Knight (Narzugon): Hell knights ride a nightmare, which are evil flying horses. In 3rd edition it says that these creatures are fashioned from soul shells that exhibited blind, impassive obedience in life. They are brave and loyal. Most of them have sworn oaths of loyalty to Asmodeus himself. They often go on missions to the material plane to destroy temples of good gods and to recover evil relics.

They have flaming lances that do a lot of damage during a charge. They almost always are on or near their nightmare mounts, and the nightmare gains skills and abilities transferred from the hell knight rider.

Hell Knight (Narzugon)

4th edition greatly expands on these creatures and change their. Hell knights were servants of the deity known as He Who Was. Asmodeus tricked them into betraying their god.

Their betrayal damned them to serve Asmodeus for an eternity. Each archdevil and duke has hell knights at their disposal. These knights yearn for forgiveness but are bound to follow the orders of Asmodeus.  

Hellfire Engine

Hellfire Engine: These are massive constructs that spew hellfire (which harms creatures immune to fire)s. They are made of cold iron that was bathed in the blood of a dozen celestials. They are often used a siege engines in the Blood War.

Hellforged Devils: These devils were created by a magical catastrophe that wiped out demon and devil armies in the Blood War. The essence of the slain devils fused with the ground and became hellforged devils. They now protect Hell itself and have prevented civil war on a number of occasions. Aurandeus, the Magistrate of Hell, commands them.

There are 6 types of hellforged devils:

Coal Devil

Coal Devil: These massive devils are made of coal with their body eternally aflame and their faces contorted into a scream. They guard and enforce the will of Hell. Coal devils can breathe fire and emit obscuring clods of smoke.

Glass Devil

Glass Devil: Made from transparent glass plates, these devils are nearly invisible spies and scouts. Their eyes are mirror-like orbs and they have extremely sharp claws.

Lead Devil

Lead Devil: Slow and ponderous, lead devils are powerful bounty hunters. They specialize in tracking down a wanted foe, grabbing them, and using their innate dimension door power to whisk the enemy away to captivity.

Obsidian Devil

Obsidian Devil: Made of obsidian and covered in in razor-sharp ridges, these devils are the police of Hell. They are unfeeling, violent creatures who obey orders to the letter. Obsidian devils like to grab victims and grind them to death on their sharp obsidian bodies.

Sand Devil

Sand Devil: These devils use their ability to turn into sand to act as informants and information gatherers.

Spiked Devil

Spiked Devil: Not to be confused with spined devils (spinagons), spiked devils are made of iron and are covered in spikes. They are guards and protectors with the ability to fire off their spikes as projectiles.

Hellwasp Devil

Hellwasp Devil: These devils live in a colony in the Garden of Delights in Malbolge. Hellwasps were once demons, but Glasya killed their master and took them back to the Nine Hells. They once served a wasplike demon lord that was slain by Glasya. The hellwasps now revere her as their queen and they consider serving her to be a reward.

Horned Beast

Horned Beast: At one time, the archdevil Geryon had a magic horn that summoned minotaurs. Once he was exiled from Hell, his power waned and so did the magic in his horn. It now summons extinct demons known as horned beasts. Horned beasts are very simple creatures similar to minotaurs. They wield greataxes, charge with their horns and when they are slain they explode in a deathly inferno.

Horned Devil (cornugon)

Horned Devil (cornugon): They are 9 feet tall, have wings and are covered in scales. They have an aura of magical fear and can fire off lightning bolt spells a few times per day. They fight with barbed whips and tail stingers.

Often, pit fiends lead armies of horned devils into battle. They are used as personal guardians of ice devils and are very loyal, a rare trait among devilkind.   

Ice Devil (gelugon)

Ice Devil (Gelugon): Weirdly enough, one of the most powerful types of devils are Ice Devils. There are actually two levels of hell that are extremely cold. This is where ice devils dwell, coveting the power of the pit fiends that are one rank above them.

They can trap you in magical domes of ice and some of them have ice spears that can slow you down.

To become a pit fiend, an ice devil must serve the aims of Hell flawlessly for 777 years. Then, it is thrown into the pit of flame for 1,001 days. During that time, it transforms into a pit fiend. It is said that only an ice devil or an erinyes can survive this experience. Exactly 1000 ice devils are promoted each century.

Imp

Imp: Just one step above lemures and nupperibos, Imps are little red winged devils with a scorpion tail. They can turn invisible and they have a poison stinger. Despite being a bit lazy and inefficient, they make great spies because they can turn invisible at will. They often act as a familiar for mortal wizards, doing their best to corrupt the souls of living beings. Most imps serve a duke named Beleth, the Witch's Viscount.
 

Assassin Imp

Assassin Imp: These vicious creatures enjoy killing helpless innocents. They have black scales and wield sharp razors. Their tail stinger inject a sleep toxin into the enemy, making for easy throat-slitting. There are more powerful versions of these devils known as Murder Lords.

Indwelling Devil: These creatures are dark spirits, unseen puppet masters that hunt down souls that escape from Hell. They frequently do work for archdevils. Many believe they all secretly serve an underling of Asmodeus called Phongor the Inquisitor

Infernal Armor Animus

Infernal Armor Animus: Devils can use a ritual to attach a mortal soul to a suit of armor. Powered by the soul, the armor becomes animate and is used as military support. When the armor is destroyed, the remains of the soul will enter into a nearby devil and heal it.

Infernal Ironguard: These devil constructs accompanied Dispater when he tried to steal the soul of Bahamut. They block attacks and knock attackers back.

Kalabon: A kalabon is a weak devil that is a heap of flesh with three legs. Here's a quote: "Blood and pus leak from its many sphincters..."

They can combine with other kalabons to form a massive, expanded mound of flesh that eats everything in its path. These creatures are remains of the Hag Countess, former ruler of Malbolge.

Legion Devil (Merregon)

Legion Devil (Merregon): These are humanoid devils, low-ranking soldiers in devil armies. They can pool their hit points together, meaning that an attacker has to do a lot of damage before they all drop at once. They are utterly loyal and fearless. In 4th edition, they were 1 hit point minions.

Lemure

Lemure: These pathetic wretches are the lowest form of devil. They wander the first two layers in large hordes, attacking intruders. In 5th edition, lemures can't die. When slain, they reform in d10 days.

It is believed that there are an infinite number of lemures in Hell that wander endlessly. Only the most evil of mortals become lemures when they die. Just one in a thousand lemures ever rise in rank. Lemures cannot speak, but they do babble and they comprehend the Infernal language.

Some devils are demoted to lemures and "marked." Those lemures bearing a mark are never promoted and are hated by other devils.

Sometimes a lemure is randomly promoted to a spinagon. Other times, a higher-ranking devil will make them fight each other to see which one they will promote.

Lillitu

Lillitu: In 4th edition, it is said that Lilith, Mother of the Succubi, promoted some devil succubi to a higher form known as a lillitu. These are more monstrous succubi, with poison instead of blood. The 3e lillitus (which I believe are all demons) have 4 tails. The 4e lillitus each have unique monstrous parts like owl heads, snakes, etc.

Misfortune Devil

Misfortune Devil: Misfortune devils are shapeshifting creatures that often disguise themselves as a relative or loved one of a victim. They encourage mortals to take unreasonable risks. They have the power to redirect attacks meant to hit them to other targets.

Nupperibo

Nupperibo: Nupperibos are extremely weak devils who are blind, deaf and unable to speak. Devils lead them using telepathy. These blobby humanoids regenerate wounds and missing limbs. The only way to destroy them permanently is to use holy water or a holy sword on them.

They are slightly higher in rank than lemures but when they are "promoted," they are technically demoted to lemure status.

• The Secret of the Nupperibos: In Tales from the Infinite Staircase it was revealed that nupperibos are not devils. They are Baatorians, the type of fiend that lived in Hell before devils ever existed! If devils didn't demote/transform nupperibos into lemures, they'd eventually grow into "Ancient Baatorians."

Yugoloths (a race of fiendish mercenary "daemons," not linked to normal demons) use nupperibos as food and accept them as currency. Yugoloths may know the secret of the Nupperibos. A few archdevils know as well.

In third edition it is claimed that nupperibos are actually devils who have been tortured and punished. Their brains were actually pulled out through their nostrils.

Orthon: An orthon is a bit like a devil ogre. They have an aura that blocks planar travel and their armor has been hammered directly onto their bodies. Their bodies are full of maggots and when they are slain, the maggots swarm and attack all enemies nearby.

Pain Devil

Pain Devil (excruciarch): These relatively weak devils are sadistic torturers. They have a fierce rivalry with chain devils, who are also torturers. In 4th edition they are found in Dis and Minauros.

Passion Devil

Passion Devil: Passion devils are shapechanging infiltrators created by Fierna, ruler of Phlegethos. Each of them is physically beautiful save for a single physical flaw. They have difficulty controlling their own passion which sometimes causes them difficulties. They have an aura that causes creatures to desire them and their fiery touch allows them to charm people. These devils are very similar to pleasure devils, listed below.

Pillager Devil

Pillager Devil: These devils scour other planes to steal souls of mortals who have passed on. They have a quota and need to bring a certain amount to their masters. Most other devils don't like pillagers because they are uncouth and malformed. They have an ability called soul grasp, meaning that they can handle intangible souls meant for other destinations.

Pit Fiend

Pit Fiend: Pit Fiends are huge winged devils who lord over all other devils. Only archdevils are higher ranked. These creatures are generals and often attend to archdukes and rulers personally. I wrote a Guide to Pit Fiends here.

Pleasure Devil (Brachina)

Pleasure Devil (Brachina): Sometimes an erinyes is promoted to become a pleasure devil, seducers who look like mortals of various races. Their specialty is corrupting agents of good gods. They can beguile a victim magically and completely control their bodies for a few moments. Archdevils use pleasure devils as playthings and then discard them.

Rage Devil

Rage Devil: The result of a dangerous experiment involving injecting demon ichor into hell knights, these 10 foot tall devils have stony skin covered in hieroglyphics of hate. When the glyphs flare, they erode an enemy's resistance. These creatures fly into joyful rages and lead legion devils into battle in the Blood War.

Shocktroop Devil

Shocktroop Devil: These devils are frontline devils for Hell's armies. They can singlehandedly slaughter a great number of the enemy. Their attacks are very straightforward - hammering away with a sword and shield. I really like the way these creatures look. The artist who designed them wrote about shocktroop devils here.

Slime Devil

Slime Devil: Servants of the archdevil Mammon, slime devils are blobs of sludge and hate. They are born in the swamps of Minauros when the souls of corrupt inquisitors mingle. They can engulf victims and burn them with acid. The slime devil will ask the victim questions. Each untruthful answer causes psychic damage.

Spined Devil (spinagon)

Spined Devil (Spinagon): These winged devils are quite weak, stand about three feet tall, and are covered in prickly spines. They are very common in Hell, flying about delivering messages while always on the lookout for intruders.

Spined devils herd lemures, forming armies for more powerful devils. Doing this well frequently gets them promoted. Some spined devils are promoted up multiple ranks all at once because they have pleased a powerful devil.

Storm Devil

Storm Devil: Found in the armies of Maladomini and Nessus, storm devils have the power to create cyclones, launch lightning bolts and let out infernal thunderclaps. They are bodyguards to pit fiends. They will disobey orders on occasion because they can't resist the temptation to score a quick kill or the opportunity to torment lower-ranked devils.

Styx Devil (Amnizu)

Styx Devil (amnizu): Styx Devils first appeared in the 1e Fiend Folio. They are ranked very high in the devil hierarchy and they have many responsibilities:

• They occupy checkpoints on the River Styx where they search and interrogate travelers.
• They search for souls to take to the archdevil Geryon. 
• They are generals of armies consisting of thousands of abishai and erinyes. 
• They are guardians of the River Styx, often referred to as the Keepers of the Styx.
• They are expected to bring new souls to the Nine Hells by using their imprisonment power and to capture mortals exploring Hell. 
• They are also record keepers, staffing the Labyrinth of Truths in Minaurous. This labyrinth holds files and records pertaining to everyone who has ever been in Minaurous and many other layers of Hell. 

Styx Devils generally believe that the rules of Hell don't apply to them due to their high ranking and special privileges. They will mess with pit fiends any chance they get out of fear and envy.

They can can magically imprison someone once per day and their touch causes people to lose one day's worth of memory. In 3e, it is said that they are often served by the wasplike advespas.

It is possible that these creatures are actually Baatorians.

Succubus: In each edition, a succubus is different. In some, a succubus is a demon. In 5th, they are both demons and devils. I covered all of this in my Guide to the Succubus. Devil succubi are ruled by Lilith, the consort of Baalzebul. Devil succubi like primarily in Dis, Stygia and Malbolge.

Steel Devil (bueroza): These devils look quite like mortal soldiers wearing armor covered in dust and cobwebs. They are blindly loyal.

Swarm Devil

Swarm Devil: Agents of Baalzebul, this devil swarm forms into an ogre-sized humanoid that gleefully does the bidding of Baalzebul. When an angel lies to its master and is torn asunder, it becomes a swarm devil that slowly starves but never dies from hunger. Swarm Devils can spew flies like a breath weapon, doing acid damage.

Vizier Devil

Vizier Devil: Vizier devils spend most of their time in mortal realms infiltrating governments. They look exactly like tieflings. They try to corrupt rulers and nations by getting them involved in an activity that starts out as a noble pursuit but becomes something grim. They have the ability to fire hellfire bolts as a ranged attack and they can imbue items with hellfire. They can speak a word of command to force creatures to do what they want.

War Devil (Malebranche)

War Devil (Malebranche): These 22 foot tall devils have wings and a black hide. They are brutes and bullies that are fanatically loyal to powerful devils. In the 4e monster manual, it says that they obey only pit fiends and archdevils.

Warder Devil

Warder Devil: Once escorts for the emissaries of Asmodeus, most warder devils now work for Dispater as bodyguards . The first warder devils were angels of protection that became too bloodthirsty and aided Asmodeus in his rebellion. They wield flaming greatswords and can teleport with a power called hell's step.

Wrath Devil

Wrath Devil: Roamers of mortal worlds, wrath devils exploit anger by goading people to give in to their hatred. The very presence of a wrath devil causes tempers to flare. Wrath devils often have bearded devils as bodyguards and are known to consort with succubi, duergar and bearded devils to hatch vile schemes with.

Withering Devil

Withering Devils: These devils cause famines and plagues by tricking mortals into depression or apathy. They each have a staff of weariness that can drain emotional energy and their gaze can cause apathy. Withering devils are careful to disguise themselves with hooded cloaks, as their appearance is hideous to mortals. 

Xerfilstyx: These insane devils have a winged, muscled upper body and the lower body of a giant slug. They are immune to the memory-draining effects of the River Styx and the experience the memories that the river has stolen from people as they swim through it. They have a breath weapon - they spew a cone of boiling blood that eats flesh and erodes memories.

Links

This article on the wizards site gives you a fantastic look at the devils and the archdevils.