Mission: Tsavo Highway

After you have to deactivate a shield that's blocking the road, you will exit onto a bridge and a giant covenant ship flies overhead. You need to get out of your jeep if you are in one and go to the right side of the bridge, then go underneath and you will see bridge supports jutting out into the canyon. Jump onto the one farthest to the left and go to the edge and turn left and you'll see a small jut out of the cliff with the skull on it.