Where did you find the "bone key"? I´m not good with those names, but the key you need to find is in a trapped room. When you enter the walls will cast Face of Death so lot´s of floating skulls will be in that room. Have you been there? You might want to check Xignal´s help site on Skull castle to see if you´ve missed something,
Xignal`s site. You can check both the graphic and the text walkthru for info. If it should prove impossible for you to get to the lich and thereefor the staff of death I´m sure Wyv will help you get one with her magic lol.