Go to the armory level in legendary. After the assistant tests you and the sergeant comes in and tells you to get in the machine, wait. You have to wait until he says "Are you going to make me say please" or something like that. Then get in and keep on looking to the right. you will see a skull resting on a box. To get it when you go by go up to the glass and Hold X by the skull. He will grab it through the glass. (I don't know what it does).