Skulltaker Halo 3: IWHBYD
Near the end of the level, after you and the Arbiter fight through the Covenant to stop Truth from firing the Halo rings, the Flood will turn against you. Fight your way back to the elevator, but DO NOT drop through the vent at the end. Instead, turn around, facing the 7 holographic rings. From here, the closest ring is 1, the farthest away is 7. FOLLOW THIS SEQUENCE: 4 6 5 4 5 3 4 (If you listen closely, the humming noise of each holographic ring corresponds to one of 7 notes on a musical scale, and the sequence of notes produced by the rings in the sequence follows the tune of the last seven notes of the Gregorian Chant from the famous Halo Theme). Afterward, head all the way back to the control console and the skull should be on the floor just in front of the light-bridge.