This skull is exactly the same as its Halo 3 counterpart: Dying in co-op forces all players to restart at the last saved checkpoint, while dying in solo play forces the player to restart the entire level. This skull can be located in the very beginning of the level Pillar of Autumn . As soon as you are able to move freely in the Cryo room, walk to the back of the room towards the two large barrels to your left. The skull will be behind these barrels.