No artifact on saccing/transforming
From bug list:
Monsters don't leave any of the guaranteed artifacts when sacced (e.g. Keethrax). This also happens if you transform the monster before killing it.
Notice that he (Keethrax) can be sacrificed on an altar (if you lead him to a dungeon level with an altar), however he won't drop any corpse or black torc, but the quest will be satisfied.