The Guardians

Looking for Guardian, the level? In Halo PC, Halo Custom Edition, Halo 2, Halo 2 Vista, 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
When a player is killed by something that isn't a player, that isn't a suicide kill, and that the game hasn't explicitly been coded to explain (as an example, a bottomless pit can be explained, but dropping dead when coming into contact with a kill barrier can't), that kill is credited to The Guardians. Kills caused by environmental objects (such as the trains in Halo 2's Terminal) and scripted environmental objects (such as the mines in Sandtrap, the cannons in Snowbound, and the towers in Sandbox) are similarly inexplicable, and are thus credited to The Guardians.

Other examples of inexplicable kills include being crushed by falling Scorpions; solid objects such as Crates (or Traffic Cones) hitting you at a high velocity; Teleporter glitches (including teleporting outside of the map, when the resulting death doesn't count as a fall); and any other unpredictable, random, or bizarre deaths.

While the Guardians' scripted appearances can be overloaded, the Guardians themselves cannot. As an example, overloading Sandtrap will disable the mines whose kills are credited to The Guardians, but The Guardians themselves aren't disabled (inexplicable kills will still be attributed to them), as they don't really exist -- the Guardians are not players, characters, or objects. The Guardians are merely a "dummy" name -- the phrases "Player was killed by The Guardians" and "I have absolutely no idea why that player died just now" are effectively synonymous.

Scripted Appearances

 * Terminal: in the form of the train.
 * Sandbox: the watchtowers.
 * Snowbound: the plasma turrets.
 * Sandtrap: the mines.

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 a short period of time.
 * Being killed by an A.I. (An A.I. can be modded into the map in Halo PC)
 * When two players melee each other at the same time in Halo: Combat Evolved.
 * Being killed by a Ghost of Halo

Trivia

 * If one overloads a map, The Guardians' scripted appearances will stop working, allowing players to explore the boundaries of Snowbound, Sandtrap, and similar maps. However, bizarre or inexplicable kills will still be credited to The Guardians.
 * There have been many cases in both Halo 2 and Halo 3 where mysterious deaths have been credited to The Guardians. This is possibly the game's only explanation for being killed by a Ghost of Halo, as a Ghost of Halo is not an (identifiable) player.
 * 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."
 * The Art of Halo 3 revealed that the Guardians may actually be a special type of Sentinel, although it may just be a coincidence.