Effect: Headshots cause enemies to explode.

Where to find: Crow's Nest

How to find: Right after the first objective, while en route to the barracks, you will fall down onto a pipe. At the end of this pipe, look over the edge to see a small space a few feet below you. You must simply drop over and as quicly as you can, pull back to land under the floor you were just on. After that, the skull will be at the end.