"Trust us. Bring a magazine."
This Halo 3 skull is found in the campaign level The Ark. It is atop one of the spires before going to the landing zone for the frigate piloted by Miranda Keyes. To get it, one needs to find themselves a Deployable Cover or be relatively good at grenade jumping. Once rounding the corner to enter the firefight before going to the cave leading up to the LZ, you should have a zone similar to the one you just fought multiple Covenant forces off of.
You should be able to get up on the rock from the left side, jumping carefully, and then get up on the big pathway. After doing so, take a long, long walk, all the way to the very last wall-like structure leading off the edge. Using a deployable cover, or grenade jump, you should be able to get up into the opening and grab the Famine skull. By getting the skull, you make 10 gs and are able to use it in Campaign. Its effects are to cut the ammo in a weapon in half, as well as what's in the clip. It also makes the ammo picked up along the way half as much.
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