He is the final, optional boss of the super dungeon Watcher's Keep. I believe it was added in the expansion of Throne of Bhaal. What was neat was that you could tackle Watcher's Keep in Shadows of Amn if you wanted to (I'm not the biggest fan of Throne of Bhaal.) Watcher's Keep has tons of great loot and provides heaps of extra experience. One of my favorite parts of BG2. The Prince of Demons is pretty wimpy as far as bosses go. I think they could have made him more powerful than the final boss of Throne of Bhaal and no one would have complained.

Thanks for the question. It is in the Throne of Bhaal expansion :)

Thanks for the comment. Yea, perhaps it could have been a bit tougher. Imo the battle at the final seal is quite difficult as well. And ofc you are in for a nasty surprise if you cast a time stop vs the DG :)

@@johngleeman8347 everyone is wimpy until you install SCS ;)