Mission: The Covenant - Skull (IWHBYD) - Inside the structure mentioned above, you have to play the Halo theme using the holographic rings near the end of the level. You’ll reach a room with these holographic rings and lots of Brutes enemies. Clear the room of enemies and activate the bridge at the far end, then kill off all the Flood that show up. When it’s time to head out, go all the way back to where you entered but do NOT drop down the shaft. Instead, turn back around. From here, the nearest ring is #1 and the furthest is #7. Jump through the rings in the following order: 4654534. This spawns the skull at the small light-bridge at the far end of the room past ring #7. Check the video starting at 06:09 for a visual example.