IWHBYD Skull
Go to the last tower. Before you get to the Prophet of Truth, there are a series of bridges with Halos. Do not jump through any yet. First, kill the Prophet and clear the bridges of enemies. Then jump through the Halos in this order, with "1" being the entrance and "7" being the end near where the Prophet was located: 4, 6, 5, 4, 5, 3, 4. Note: You can also use the longer sequence of 4, 5, 6, 5, 7, 6, 5, 4, 1, 2, 3, 2, 7, 2, 1, 3, 4, 6, 7, 5, 4, 5, 3, 4. This is much easier in Co-op mode with three or four people. If done properly, the Halos will start flashing. If the Halos do not flash, revert to your last saved game. The skull is near the Prophet's ramp. Its description reads "But a dog beat him over the fence." This skull unlocks hidden dialogue when activated. If this is the thirteenth skull you have located, it will also unlock the Hayabusa Head.