Zombies

Were you looking for The Flood, a zombie like species inhabiting the Halos or Infection, a multiplayer gametype in Halo 3?

Zombie (or Zombies, Pandemic, and Dawn of The Dead) is a player-created game variant originating in Halo 2, although variants are also played in Halo PC. It's roots are now untraceable, and has become so popular that it has been translated into Halo 3 as Infection, with a few minor edits and many more custom game tools. The basic rules involve two teams: a small starting green (or blue in some cases) team and a relatively large red team. There are no shields; red sports Shotguns and Pistols and green (the zombies) sports Energy Swords. Whenever a red team member is killed, they must switch to green team. The ultimate goal is to last as long as possible without getting killed by the Zombies, and becoming one themselves.