Location: After the gondola ride, clear the area ahead of enemies and then activate the piston in the adjacent area. Drop down to the area below and look for a stack of five boxes. Jump to the box at the top of the stack and then over to the nearby walkway. The skull is in one of the alcoves along this walkway, floating above an enemy’s remains.
This skull triggers the “Mythic” difficulty level. Enemies are even harder to kill and deal more damage. 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.