Find and claim this skull when playing on Normal, Heroic or Legendary difficulty.
Mission: Sierra 117
Effect: "Death carries a heavy price..." -- Instead of only losing 25 points when you die, you lose 75. You also restart from the beginning instead of any checkpoint.
After Freeing Johnson and killing all the covenent that come in the phantoms, walk to the Pelican that comes to pick you up. Face the cargo bay but instead of hopping in, turn left and follow the path. Once you hit the wall jump up to your right, eventually reaching a walkway. Walk all the way to the end, take a left, then a right, then at the end of this little bit to the right is a dark corner. Turn your light on and you should see the skull. If you cant see it you're probably standing on it.