Halo 3 cheats page 7

[partyjock]

[FONT='Verdana','sans-serif']Legendary difficulty settings. You must start from the beginning of the mission. You will not be able to see the skulls if you load from a checkpoint, unless doing so after you die. In Co-op mode, as soon as one player picks the skull up, all players will receive the achievement. Not all skulls earn an achievement; only nine of them do so. To use skulls, press X at the lobby screen. This will give you a bonus multiplier for co-op and go towards earning the Guerrilla, Demon, etc. achievements.[/font]