When it goes right, it can be hilarious and fun. But very often, it goes horribly wrong and can end campaigns and even destroy entire gaming groups.
The Lady's Man: One guy was playing a really evil character who always hit on the character of a female player. Her character finally had enough and slit his throat in his sleep. We had already established the rule that if somebody did this, they were just dead. No roll! He was crestfallen, to say the least.
That Guy: We had a new player join our group who liked to make drow assassins. He'd kill a PC, and then we'd kill him. Then he'd make a "revenge character" and come back for more killing. He got kicked out of the group after about two sessions.
The Rift: There was an epic battle between a psionicist and a dagger thrower. The psionicist put up an inertia barrier, which caused projectiles to lose their inertia and plop to the ground.This of course made for a hilarious battle. All the dagger thrower was good at as throwing daggers. It was a stalemate until the dagger thrower put a bag of holding in a portable hole, creating an explosive rift to the plane of fire that killed both of them.
The PVP Dungeon: Once in a while, we'd play a special one-shot where we'd go through a competitive dungeon. We'd all race through deadly trap-filled catacombs alongside NPCs who would drop like flies. Those of us who survived would face off in a mass free-for-all in the final room of the dungeon. The last survivor got awarded special powers (basically a special "prestige class", before those existed).
The Incident: Last year, I ran The Dancing Hut of Baba Yaga. The heroes were discussing plans, and a party member suggested something that the party wizard didn't like. The wizard hit him with a spell and nearly killed him. Just minutes later, the heroes tricked the wizard into leaving the hut, and then deserted him on the evil plane of his greatest enemy. The campaign pretty much ended right there.
Causes of Interparty Combat
Let's See Who's The Toughest: If you have a few competitive players who are both trying to make a powerful character, chances are that they'll be comparing themselves to each other. A little good-natured ribbing could slowly turn into a feud, with everyone else watching on the sidelines piping up and openly speculating on who would win. Sooner or later, these two are going to butt heads to see who is better.
Domination: This is something that really needs to be used carefully. In D&D, especially in 4e, some bad guys can dominate a PC and force them to attack their allies. As DM, you're probably going to dominate the person who does the most damage, right? That means that over the course of the same campaign, the same player is going to be put in a situation where he or she has to hack into their allies. This player will likely become unhappy and the others might even be annoyed at how much damage their ally is doing and begin to question if their character is made correctly. From there, it's not much of a leap to go from forced inter-party combat to voluntary inter-party combat.
Magical Alignment Change: This actually just happened in my planescape campaign. Bidam the dragonborn fighter was hit by an energy field while exploring the corpse of a dead god, and he went from Chaotic Good to Lawful Evil. Sometimes, when a player is put in this situation, it's really weird for them and they don't know how to play it. What would an evil person do? Do you just do the opposite of what a good character would do? Honestly it doesn't seem like there's a lot of upside to even including alignment switches in your game, unless it is to affect NPCs. You're tampering with a PC in a way that might make the player not want to run their character anymore.
Party Thief: Here's the classic. A thief joins the party, and starts stealing from the party and secretly hoarding treasure. You probably know that there is no quicker way to incite player rage than to try to steal from other people in the party.
Claiming a Magic Item: The party comes upon a truly awesome magic item. Two characters really want it. A lot of shouting, rolling and snatching takes place. Player greed is partly the issue here. Also, this could be a case of the DM not handing out enough magic items - players are starved for them and become rabid dogs.
Player Decides to Sabotage the Game: This really does happen. Sometimes, for whatever reason, a player decides that they don't like this campaign and they purposely try to ruin it. I've seen it happen many times. I've heard people brag about it. It's about as pathetic as a D&D player can get. These toxic people need to be ejected when discovered. Just retcon their stupidity.
Using Skills on Each Other
If you can juggle this kind of situation without boring the group, do it. It's awesome. There's something very exciting about a game where players genuinely don't know what the others are doing. If you have players who aren't going to get distracted or disinterested while you handle other people, definitely give it a try sometime.
Why Even Bother?
The thing here, though, is that there's probably going to be hurt feelings involved. The person who loses is probably going to lose due to bad dice rolls, and that is a tough pill to swallow. If the winner is going to gloat, then you will have a real life problem on your hands.
To me, the best time to have this happen is when a player quietly comes to you and says that they want to change characters. You can cook up a scheme where the character betrays the party and battles against them, knowing that the character is going to lose or die.
Another good time to do it is in a one-shot competitive adventure, like I've described above.
The most important thing when thinking about inter-party combat is that you need to know your players. I don't want to be condescending and throw around the word "maturity", but really that's what it comes down to. Is your entire group mature enough to handle the consequences? Are they going to be bitter and ruin your campaign over this?
Running an Inter-Party Combat
- Make sure this is not an out-of-game issue spilling over into the game. If it is, then just stop the game and work it out before continuing.
- Every roll must be witnessed and declared. Do floor dice count? Cocked dice? Make sure those rolls are on a flat surface, out in the open. Never is the temptation to cheat so great.
- You must look up every rule as you go. You cannot blow a call in an inter-party combat. Things might get intense, so you'll need to be double-checking as a player might broadly interpret a rule to gain an advantage.
- Keep the rest of the party in initiative order. They may want to jump in, heal someone, or try to break it up.
- It is vitally important to establish who is doing what, when. People will be excitedly shouting things out, lots of words and dialogue coming out at once. Use initiative order as a guideline, as well as common sense. Make it clear what is taking place, and in what order.
Inter-Party conflict doesn't have to involve a fight to the death. It can just be to unconsciousness. It cold even be to "first blood" (half hit points..?).
The conflict might not even be a physical battle. It could be a foot race, a grappling match, an archery contest, a spell duel, a singing competition or a dance-off (which would be hilarious). Or my personal favorite: A drinking competition.
The safest way to explore inter-party combat is in a classic setting: The Arena. You can run a big arena battle, which is to unconsciousness, not death, and you can have other NPCs in there, too. This will create a nice buffer so that your players aren't directly pitted just against each other. The random nature of the battle will give the players an out, so that the loser could blame the setting for the loss.
It also allows you to add some cool environmental wrinkles - pit traps, projectiles. Heck, maybe even do a chariot race! There was a video game that made a very lasting impression on me when it comes to arena fights. It's called Shadow of Rome. The battles in that game are so crazy, chaotic and fun that I've stolen from it many times for D&D.
Bottom line, inter-party combat needs to be handled really carefully. It can add tons of drama and legendary stories, but it also can kill your game and your gaming group. Tread carefully!