Difficulty: Legendary
Level: Regret
Word: Assassins
Effect: All enemies in game are permanently cloaked, AI cannot see them and so are no help at all.

Just after you get off the gondola, and go in the structure, there is a big block of stone, jump on that an you can get on a ledge that leads out to the Covenant turrets, facing back towards the structure, jump up another level around the right side of the structure, It is pretty straight lined from here. You should find the Skull on a ledge when you make it around. No visible change to the HUD, BUT all enemies become invisible.