For Halloween (and until 3:00 on November 1, Marty has heard, but what time zone I don't know), there's a zombie game for Halo3 players. When you go to matchmaking, one of your options is zombie.
Zombies have swords, others have guns, and when you "die" you become a zombie. The game's over when time is over or when everyone's a zombie.
Marty and Holly tried to play two at a time, but there were problems. They took turns.