The Guardians

Summary
In both Halo 1 and Halo 2, when a player in multiplayer is killed inexplicably (i.e. by a physics glitch, by the environment, or by an 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), 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. Also being killed in & out of maps by unmanned vehicles has been known to cause this and has resulted in an unexplainable maniacal laugh being heard on occasion. Another more simpler way to see the message is in the level Terminal. If you stand in front of the Train and let it crash into you the sign will pop up at the corner of the screen You can do this with a Warthog as well.