Go to Fort Redman, which is located along the riverbank southwest of Fieldhouse cave. Once inside, go forward until you see a bridge. The bridge lowers when you get near it, but a trap will also be sprung. Cross the bridge and continue straight. You should see a statue on a ledge. If you have a high enough Acrobatics skill, you can jump up to it. Behind the statue is a skull that is about four times the size of a regular skull.