Location: This skull can be grabbed during the second and last gravity lift trip. The second gravity lift is beyond the circular room that has an Energy Sword on a pedestal at its centre. Step onto the lift, and look up completely. Hold the X button while traveling up; you’ll eventually reach the skull. Master Chief will grab it, provided you are holding the X button when it appears.
With this skull active, all enemies have active camo. This effect remains active until you turn off your Xbox and reload the game. To re-activate this effect afterwards, you’ll have to collect the skull again.