Cheat List
These cheats are ONLY for the Custom Edition of Halo: Combat Evolved.

Open the properties window of the Halo shortcut and add “ -console –devmode” after the target. Start the game with the -console -devmode command line parameter. Press ~ during game play to display the console window, then enter one of the following codes to activate the corresponding cheat function. Codes do not work in online games.

Effect : Code Instant deaths for enemies that see player: cheat_medusa <0 or 1>
Set game speed game: speed <0-20>
Clear console window: cls
Unlimited ammo and no overheating for weapons: cheat_bottomless_clip <0 or 1>
Unlimited ammunition: cheat_infinite_ammo <0 or 1>
Super jumps: cheat_super_jump <0 or 1>
Possess touched character: cheat_bump_possession <0 or 1>
Invincibility: cheat_deathless_player <0 or 1>
Spawn Warthog: cheat_spawn_warthog
Spawn all power-ups: cheat_all_powerups
Spawn all vehicles: cheat_all_vehicles
Spawn all weapons: cheat_all_weapons
Teleport player to current camera position: cheat_teleport_to_camera
Quit game: quit

Alternate ending sequence:
Successfully complete the last level of the game in campaign mode under the legendary difficulty setting. The new ending features a human and alien fighting hand to hand. The human will say "Its over", and then they hug because they know the explosion will kill them both. Also, the FMV sequence of each battle scene will be slightly longer.

Alternate dialogue:
Playing under the legendary difficulty setting introduces new dialogue not heard in other modes. For example, after being massacred by a force of Grunts, Jackals and Elites, one of the grunts asks, "Can I have his helmet?". Also, your fellow Marines seem to be a bit more chatty (as well as profane).