At 24% just before he fd's I decided to ht him and kill it just to see if it could be done. It bugs the task if you do that. It won't let you loot the skull saying a guard is still around even if you killed one completely and the other one is fd'd just FYI.
Guards Remains of Master Marthor. To open the remains you have to dps the 2 masters to around 25%, then they FD and you are free to open the skull. This is for the Evil Afoot quest from Phara Dar. A subquest for the Grand Illusion quest.