It is a large "grim reaper" type creature, at the center of the Royal Tomb in the Ancient Necropolis. This area is only accessible after you assemble the Royal Tomb key using cursed items from the surrounding area which have been purified. The area is rather difficult, and at this time I'm not sure what, if anything, this creature drops. Myself (level 42) and 2 level 50s went into the area and, with much trouble, finally killed him. However, he didn't seem to drop anything special. If this indeed is the final part of this hidden quest, it doesn't seem to be finished.