Fairfax Tomb is an area located underneath Fairfax Castle, accessible through the library. It is infested with hobbes, balverines, and bandits which you must defeat in the Castle Fairfax quest. At the end of the tomb is a room in which the Potion of Transmogrification lies. You must decide here whether to change your gender or to leave it as it is. To leave the room will stop the potion from working. There is an exit to Bowerstone Cemetery out of this room too.