The Guardians

Summary
In both Halo 1 and Halo 2, when a player in multiplayer is killed inexplicably (ie by a physics glitch, by the environment, or by AI in Halo: Custom Edition), the message reads "_player_ was killed by The Guardians". There are also ways to do this among players (guardian glitch). For example, aim at a teammate's head with a battle rifle (or a pistol or Carbine) other weapon, right trigger and Y simultaneously until he dies. Another way is when a player is killed by a melee attack from another player who died just as they did the melee attack.