The Guardians

Looking for Guardian, the level? In Halo PC, Halo 2, and Halo 3 when a player in a multiplayer map or game 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". This is sometimes triggered when trying to get out of a map, but it is not limited to that. Things such as scenery, objects, and even teleporters can cause a death by the "Guardians". The same goes for being killed in and out of maps by unmanned vehicles (this may not happen in Halo: Combat Evolved, as it will say "(Player) was killed by a vehicle".

Basic Theory
It seems to be that the Guardians are a form of security, that is, they protect the map and the game from the players that attempt to (and also accidentally) go past the level boundaries, or glitch the map in some way and sometimes if you try to super bounce on levels you go through the floor and die. This, if proven, would confirm a fact that shows that the normal map boundaries are just blocking off the outside edges, and preventing the player from violating the Sealed World Rules (a set of rules dictating level geometry and physics). Therefore, if the Guardians could somewhat be "glitched"; removed from active play, then these Rules could be exploited. However, the Guardians are well designed, in that there are very few manageable actions that a player can do short of hard hacking the game that will disable them.

Guardians should basically be assumed as a form of A.I. that shows that you were killed when the map stops the players from exiting the map as in Sandtrap and Snowbound (Halo 3), or when redesigned part of the map killed you such as the trains in Terminal (Halo 2) or any odd forms of deaths like falling Scorpions, solid objects such as crates hitting you at a high velocity, teleporter glitches or any other deaths caused by unpredictable events. The Guardians can be overloaded if they are in the form of a turret or mine, but a secondary wall has been placed outside most maps to prevent players from getting a little too far. This wall cannot be "glitched" off.

Examples

 * Falling to your death.
 * Being splattered by an object of its own accord, such as a flying fusion coil, or the occasional empty vehicle. These can happen through means of relatively rare circumstances and explosions.
 * Being knocked off a ledge by an object.
 * Forge games often lead to multiple Guardian kills, due to its physics manipulating nature.
 * Being weakened by enemy forces, only to be killed by something brought about by the above.
 * Being outside of the level boundary for quite some time.

Trivia

 * Most equipment/power-up types, including Bubble Shield and Deployable Cover do not alter the effect of the Guardians and you will still continue to take damage as normal.
 * If one overloads a map, the Guardians will stop working, allowing players to explore the boundaries of Snowbound, Sandtrap, etc.
 * There have been many cases in both Halo 2 and 3 where mysterious deaths have been caused by the Guardians. This is possibly the game's only explanation for being killed by a Ghost of Halo.
 * A glitch in the Halo demo causes the player to spontaneously die. When this happens, a message appears saying "You were killed by the Guardians."
 * Though unconfirmed, there could quite possibly be an Easter Egg, in Halo 3, in which you can hear evil laughter via your headset after killed by The Guardians. This was heard and accounted for by JayQs1919 when playing a custom game with a friend.
 * In the popular machinima, Matchmaking, the Guardians are represented as a select few people (with actual human voices, possibly selected or working for Bungie) that have complete control over every aspect of the online world and are responsible for banning people from Matchmaking, usually for specific reasons.
 * Being killed by the guardians when outside of a map may be one of the most mysterious ways to die the game as you just fall on the ground without contact with any other person or object.