Mission: The Covenant

Effect: "But the dog beat me over the fence" - Unlocks bonus dialogue throughout the game. For most, this is the last skull, so this gives you the Hayabusa Helmet as well.

To get this, get to the room where you "fight" the Prophet of Truth. Let the Arbiter kill him, turn around, and kill all the Flood here as well; makes it a LOT easier. With all obstacles out of the way, you need to jump through the Halo holograms here in a specific order, listed below:

4 5 6 5 7 6 5 4 1 2 3 2 7 2 1 3 4 6 7 5 4 6 5 4 5 3 4

When you jump through the final hologram, they all light up in a sequential pattern. The Skull will be at the end waiting for you right before the energy bridge leading to Truth's corpse.