To spawn him, kill all the skeletons around the tower. You'll see the message "You feel a presence nearby" when he's spawned. He shouldn't aggro until you approach the inside of the tower, so if your last group of mobs is behind the tower, you should be okay to circle around to the front to kill him. Watch for knockback by placing the tank with their back to the wall or the tower itself. Be prepared for something like a harm touch.