The Ravenous Defiler is one of the two optional bosses beneath Black Castle. He is found by going downstairs from the entrance of the castle on your first left or right once zoning inside. Follow the path inside the cellar until you come into a tunnel in the wall; then taking the path to the right into a group of Snake looking beasts.
Make sure to clear the room first because him with an add could be a challenging fight. However, the Ravenous Defiler himself, is a weak mob and an easy Tank and Spank to kill.