From Halopedia, the Halo wiki
This article is about the gameplay aspect. For the Forerunner machines, see Guardian. For other uses, see Guardian (disambiguation).
"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 "<player> was killed by The Guardians" is shown. The message appears in Halo Combat Evolved, Halo: Combat Evolved for PC, Halo Custom Edition, Halo 2, Halo 2 for Windows Vista, Halo 3, Halo 3: ODST, Halo 4 and Halo: Reach.
Guardian deaths are caused by the game being unable to explain the death of a player.
In Halo: Combat Evolved, every object is assigned ownership. As an example, any bullets generated as a result of a shooting player would have their ownership attributed to the player who fired them. However, in cases where a player is killed by objects that have no ownership, the resulting death will be credited to the Guardians. This theory can be easily tested by having a player fire a rocket or any other slow moving projectile at another player and exiting before the impact. Since the player has left, the rocket will no longer have a valid ownership attribute and the resulting death will appear as a Guardian kill.
Melee deaths in Halo: Combat Evolved are subject to the same system. If two players simultaneously melee each other, one of the deaths will be attributed to the Guardians since logic in the game prevents a melee being attributed to a player who's awaiting respawn.
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 <Player> died just now" are effectively synonymous.
On Bungie.net, individual players have a Service Record; a definitive page for nearly every kind of statistic there is in the game: kills, deaths, ratios, wins, etc. On these pages, one can find the Guardians listed as a cause of death under the Deaths section in most games that are tracked by Bungie.net (Halo 3, Halo 3: ODST, Halo Reach). The circumstances of the deaths are as they usually are; strange occurrences, explosions, etc. however it is interesting that the Guardian deaths have their own section and ratios as they are more common in the more recent Halo game releases than they were in Halo Combat Evolved or Halo 2. In Halo 3 and Halo 3: ODST it is actually possible to kill with the Guardians, as the circumstances can be initiated by a player, and on the stats pages a player has a kill/death ratio with the Guardians listed as a "Tool of Destruction".
The following appearances are "scripted" in the sense that there are specifically coded animations and interactions in the levels; there are no special in-game messages, however, and these are all listed as Guardian kills in gameplay statistics.