Baal's Eye is a quest item in Diablo II. It is only available after version 1.11, on Battle.net.
This is the so-called "eye" of the Prime Evil, Baal. It is achieved when a high leveled player is trying to complete the Pandemonium Event. The player must travel to the Forgotten Sands and battle and destroy Uber Duriel, who will drop the item on his death.