The skull can be found on the level The Ark (Level).
It is represented by a blue circle with a skull in the middle. The skull has a smaller skull on its forehead with smaller skulls in the background.
Effect: Trust us. Bring a magazine. Dropped weapons will have half as much ammo or energy as normal.
To reach the skull: When your path branches off into two sections, take the right path, as you would if you were completing the mission, and you will notice a structure above you/to the left of you. You need to scale the rocks to the right of the structure and then travel across it so that you pass 3 of the large support beams extending into the cliffs. Inside the fourth beam is the skull. A well placed and timed grenade should help you get up there quickly.
You can extend your jump by using deployable cover close to the wall just before the floor slopes downward, then at an angle run jump towards the skull. It's easier than trying a grenade jump.