Where is the citadel skull?

Answer 1

The skull on The Covenant, in the citadel, is a bit of an annoyance to get. First get to the huge room with all the glowing rings. DO NOT JUMP THROUGH THEM! If you jump through a single one, you have to start over, or at least from the checkpoint. After you have cleared all the enemies and {SPOILER} killed Truth {END} you have to jump through the rings in a certain order. Make sure you've killed all the Flood, because if you die and return to a checkpoint where you have already jumped through a ring, you're screwed. Once all the enemies are gone, jump trough the rings in the order:

4 - 6 - 5 - 4 - 5 - 3 - 4

I believe the order is because of the notes the rings play [halo theme] when you jump through, though I am not sure.