WAIT!!!!!!!
I am confused. I believe you CAN miss it. Now that I remember, I actually re-loaded a save and completed all the side quests, but without the skull I could still get the necromancer, I just imagine even with him you'd still need the skull, and you'd probably need to complete that quest to do so.
I honestly don't know. :'(
Sorry for the confusion.
No need for panic
The first time I also miss the Skull (don't find it) and when I had my Battle Tower, the Necromancer there give me a Skull !
