Warlord

Lots of chaos, everyone fights to the very end, and no one loses!

Fighters: The more the merrier, but works well for small groups as well.
Regen: A fighter who is killed regenerates and joins the team of their killer after counting down from 3. Fighters cannot attack or be attacked until they have stood up.
Teams: None to start with, one in the end. See below.
Rules: Starts as a cutthroat, with everyone on their own team of one. As described above, whenever a fighter is killed they join the team of their killer. If the warlord of a team is killed, everyone who had been on their team joins the warlords new team (or not, see options below). Thus, if at the start of the battle Conan kills Elminster, Elminster regenerates and joins Conan's team. If Huma comes along and kills Conan, Conan regenerates and both Conan and Elminster join Huma's team. If Rand then comes along and kills Conan yet again, Conan is now on Rand's team, but Elminster remains on Huma's team (as Huma is now Elminster's Warlord). To minimize confusion, all fighters should call out the name of their warlord as they fight, so their opponents know what team to join if they're killed. Additionally, whenever a warlord is killed they should shout out "[dead warlord]'s team is now [new warlord]'s team," until everyone gets the message. At the end of the battle everyone should be on the same team, whose warlord will be the only person who was never killed.
Optional: Warlords do not bring their whole teams with them. This can reduce some of the chaos by not having sudden mass loyalty changes, but it can also cause additional confusion when an ex-warlord is fighting their "own" team, or backstabs them to bring them over to his new team.