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, but no one must be in the Warthog, ways to do this, is to drive a Warthog at top speed and get out before hitting someone.

There is also a third way, doable in Halo 2. Dual wield Plasma Pistols and have any weapon on backup. Charge both plasma pistols, and a split second after you release the charge, press B. If done correctly, the second pistol will disappear instead of drop, and you will still be holding the two pistols on everybody else's screen. Pull out your secondary weapon, crouch(to prevent shooting at yourself, sometimes you do that), and open fire on a player. The bullets will seemingly come out of nowhere, and when you kill the other player, it will say "Killed by the Guardians" on their screen.