Have you actually killed him? Because that is what you have to do. You have something like 10 seconds to kill him before he disappears again. One of the strategies going around (and i haven't tried this btw) is to get freezing weapons that freeze him so you can keep attacking him until he is dead.
Another aspect of that tactic is, if you unequip your melee characters weapons and attack with fists. That way, you get loads of rares and uniques from him, before he dies.
However, to get the final item you actually need to kill the ??? monster. If you have done that, then i have no idea. Is there something else that needs to be done? I think there is a strategy on this site somewhere...check the walkthrough / set items. Might be there.