Thank you, and I'm now going to abandon this thread. I'm afraid of hearing any spoilers.

To the OP:
Good luck on the Slenderman, but if he's a horror figure, I'd suggest against full stats for him. A good part of proper horror (to me), is the unknown. If the PCs try wildly flailing and attacking, and suddenly discover that he's got an AC of X, and is vulnerable to Sonic, he's suddenly going to be a lot less scary.
Especially if next level the wiz/sorc gets the feat to turn all his feats into sonic spells.