Speedrunners and glitch-hunters have discovered how to play as almost any character in Halo 2, ranging from bipeds such as grunts, elites or brutes to holograms, such as Cortana or the Heretic Leader. It even allows players to play as vehicles, including warthogs and phantoms! The glitch, called Arbitrary Unit Possession, works by cleverly abusing Halo 2’s vulnerable respawn mechanics and makes for some hilarious results.
This glitch was discovered by speedrunners Harc The Shark and MisterMonopoli late last year and has since been perpetually explored and mastered by speedrunners and glitch-hunters alike, allowing players to play as any character or vehicle that you would ever encounter in-game, though some units are harder to possess than others. The workings of this glitch are rather simple. When a player in co-op dies, the game creates a model of their character in a place out of sight, referred to by players as the “default spot”. The game repeatedly tries to teleport this character to the other’s position once the game deems the alive player to be safe from combat. Once the character is teleported, it simply ascribes control of the model to the respawning player. The trick works by confusing the game into thinking another unit (an enemy, a marine, a vehicle, etc.) is the model to be teleported and ascribed control to.
How it works: Every item in the game that spawns, fired and thrown projectiles included, has both a salt and an index value. The index represents the number of loaded items at the time of the spawn, while the salt is a number that increases by one with each new spawn. In order to trick the game into giving a player control of the wrong unit, you must load in a unit that has the same salt and index as the character model that appeared in the default spot, while that character is dead. The game will attempt to teleport the model to give the player control of, but will select the newly spawned unit instead of the Master Chief/Arbiter model in the default spot. But how do you get a unit to spawn in with the same salt and index as the default spot biped?
How it’s performed: In order to do so, players will have one of the two players die, while the other enters a “pinch”. A pinch is when a player is wedged into a tight spot that leaves no room for the other to spawn, usually accomplished by crouching under a low ceiling. The game will see that player as safe and will want to teleport the respawning biped to that player but will be unsuccessful. If you then restart the mission, the game will continue to try to teleport a character who has those same salt and index values until a respawn is performed, though the salt and index values of all items and units will have reset when the level restarted. By having the same player die as before, then having a new unit spawn in with those same salt and index values as before, the game will respawn the player as that unit. Since a player death on legendary will reset both players to the last checkpoint, this glitch can only be performed on heroic or lower.
This can often be very tricky to perform, as any bullet fired by anyone will increase the salt, which can mess up the value you are attempting to reach. For this reason, this glitch is most easily performed early in levels that don’t spawn you in action from the immediate start. You’ll also be required to know the salt/index values of the unit you want control of, which is unfortunately very hard without being able to read the game memory, but speedrunners have put together numerous step-by-step guides that will let you turn into particular characters easily. With experience, you can estimate these values based on how many units spawn in the level up to that point, though that’s best saved for once you’ve mastered the basics. The easiest for beginners would be a flood form on The Oracle, a hunter on The Great Journey and a brute on Uprising. Those hyperlinks will take you to video tutorials. Very experienced players have been able to spawn in as Cortana (who strikes a T-pose if you try to jump) and even warthog machine gun turrets that the other player can operate.
The neat thing about this trick is that you preserve the AI characteristics of whatever unit you are playing as. If you spawn as a marine, you can pilot pelicans. If you spawn as a brute, you can wield Tartarus’ gravity hammer. If you spawn as a flood, your character will - against their will - attack your friends. It is absolutely wild and a stupid amount of fun. For those starting out, definitely follow the guides in the paragraph above for the simpler unit possessions, then branch out and start trying your own permutations to see what sorts of characters you can spawn as!
Source: Halopedia
