You can find a crystal skull in Broken Valley, in an area you should be able to handle about level 11 or 12 (at least based on D2:ED). If you miss that skull, you will be given one by your necromancer in the battle tower. The first one is
in a mine, with the entrance at the back of a camp where a lone, fatally wounded miner will call out to you when you get near, and describe the attacks in the mine. The camp is to the far west of Lovis' tower, a little south of the fortune teller.
