Hehe. Sharp pointy teeth is exactly what I thought of too. I like the spider climb idea as well. You could really make him seem as if he was undead himself when really he's just a gnome necromancer with some really terrifying habits and grooming...
You enter an enormous room at the center of the long abandoned temple to Pelor. The sharp acrid tang of fresh blood mixed with a dry, dusty, smell fills your nostrils. There is an aisle down the center of the room straddled by giant pillars that reach up into the impenetrable darkness of the vaulted ceiling far above. On either side of the aisle are rows of benches that are knocked askew in random directions and are in various states of collapse. At the end of the aisle is an altar, and next to it a ceremonial basin filled with a dark liquid.
The thick layer of dust that covers everything is disturbed by shuffling prints tracked through the room coming from and leading toward every direction imaginable: across the floor, over the benches, up the pillars, down the walls. Upon closer examination you notice that the tracks are made with small bloody handprints.
You suddenly become aware of a shuffling sound, but the reverberance of the great hall makes it impossible to pinpoint the source. In the dark of the ceiling above you can just make out a rustling movement. Before you have time to react, down one of the pillars, upside-down, crawls a small gnome with pale, emaciated, features and deep, dark, eyes that hold a deep and disturbing intelligence. He holds a dismembered arm in his hand, which he is gnawing on with sharp, pointed, teeth. The arm falls from his hand and slides to the ground below. "First I kill you in a slow and agonizing fashion...then the suffering begins." He snarls at you and blood drips from between his teeth. With lightning speed he races back up the pillar and into the darkness above. You hear the ominous sounds of chanting echoing from the ceiling of the chamber...
Perhaps the most terrifying part of the encounter will be when they realize that he isn't undead at all, just stark, raving, mad!