Diablo/Zhar, the Mad

At catacombs level 8, find Zhar, the mad sorcerer that thinks he's powerful. Talk with him and he will hand you a random spellbook. However, he will warn you to not touch the bookcase, since the wisdom contained there is only for him.

To anger him, grab the book on the bookcase and he will start attacking you. Ranged attacks are the safest way to kill Zhar, so if you're using a Rogue or Sorcerer, you will have minimal problems. Warriors may have a tougher time, but you may be able to drive him into a corner occasionally and get some extra hits in. When dead, he will drop a random item that can be normal, magic, or unique.