Instructions
Progress through the single-player or cooperative campaign until you enter the Ark Citadel building to confront the Prophet of Truth. This is the series of events that happens immediately upon finishing the battle in which defeat two Scarab war machines.
Meet the Prophet of Truth. Defeat all enemies present, moving back toward the room from which you first entered the Ark Citadel.
Note the rings that line the walkway. Label them in numerical order, starting with the rings in the entrance room and ending with the rings in the Prophet of Truth room.
Jump through the fourth ring, followed by the sixth ring, then the fifth ring, then the fourth ring, then the fifth ring, then the third ring, and finally, the fourth ring. You must jump through the rings in that specific order. If you jump out of order, begin again.
Move to the Extend Bridge near the Prophet of Truth room. The skull will appear there once the ring sequence has been completed successfully.