HaloRadar: Guide to Every Skull

Mission: The Covenant

Effect: All enemies promoted one difficulty rank. Basically, every foe is tougher and smarter.

Instructions:As soon as you get a Hornet (and see the chapter title "If You Want it Done Right") fly around the bend and land on the first structure. Lying inside the part of the building that juts out over the water is the skull. Pretty straightforward.