The Guardians

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"The Guardians" is a placeholder name shown when a player's character in a Halo game dies of unknown causes. When a player dies under bizarre or unknown circumstances, the message " was killed by The Guardians" is shown. The message appears in Halo PC, Halo Custom Edition, Halo 2, Halo 2 Vista, and Halo 3.

Explanation
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 " was killed by The Guardians" and "I have absolutely no idea why  died just now" are effectively synonymous.

Scripted Appearances
It should be noted that the Guardians' scripted appearances are actually a lack of scripting: the developers of the Halo games chose not to add code explaining the deaths that could result from the appearances.
 * Terminal: in the form of the train.
 * Sandbox: the watchtowers.
 * Sandtrap: the minefield out of the level's boundaries.
 * Snowbound: the plasma turrets at the boundaries.
 * Standoff: the gate behind one of the bases.

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.
 * This does not include objects that are being actively manipulated by a player in Edit Mode.
 * In Halo: Combat Evolved, there is a separate message for unmanned vehicle kills.
 * 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 boundaries for a short period of time, or touching a kill barrier.
 * In some cases, touching the ground outside of a level for a total of between one and five seconds results in a death. The amount of time varies from map to map (and possibly between areas of maps), but it is cumulative -- if a total of five seconds can kill a player, then a player can also die by touching the ground outside the level for two seconds, jumping onto a vehicle, and then touching the ground for three more seconds.
 * Being killed by an AI-controlled NPC (they can be modded into maps in Halo PC).
 * When two players melee each other at the same time in Halo: Combat Evolved.

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 Trial 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 an enemy cut from the enemy, called a Guardian Sentinel. There is, however, no known or probable connection with The Guardians.