Find the overshield on the blue side. Then look for the wall with the blue triangular shapes on it. There should be a pillar in-between the shapes. At the top of this pillar, to the right in an alcove is the skull. Only accessible via Forge. To get the skull down use a grav lift, but make sure you see where it falls otherwise it will be lost.
- The Citadel Skull is engraved with what appears to be the symbol for the Mantle of Responsibility.