He appears as a giant, violet, glowing skeleton with a distinctive sword. He can be found wandering in the room just beyond the Chamber of Summoning. It is highly recommended to kill the first three groups before engaging combat with him.
There is not much to say about how to fight him since he's very easy but it's worth mentioning that if there is no one to tank him, he will start shooting arrows at the group. Healers should know that he has the health worthy for a boss beyond his level so be warned to preserve your mana.