Unknown Skull In Halo 3

In Halo 3 there is a unknown in the level Cortana. It is in the room [before you get Cortana] where the fusion reactors are. There is a ledge where you can jump onto the level above the main door [it is recommended that you kill all flood in the area before you do this but you do not have to] and you might have to grenade jump to get up there. Look around the upper area until you find a skull with bones around it. Some people say if you didn't get the blind skull from the first level,Sierra 117, its a blind skull and you will be able to use the blind skulls affect. Also, if you carry it throughout the whole level you start out with a Spartan Laser in the next level, Halo.