The bone you need is actually as the other poster said. It is a tooth from what the imp called the bony monster. If you look on the map you'll see a giant skeleton of a monster in a room. You have to click something is particular and i believe it is a torch to unlock the door. Just to prepare you, the bony monster is a giant spider. <img src="/ubbthreads/images/graemlins/alien.gif" alt="" />
