Diablo/Warlord of Blood

At hell level 13, find a book and read it. It will tell you something about a war and a powerful demon called Warlord of Blood. If you don't find the book, just find him and talk with him. He is somewhere here.

Simply kill his minions. After talking with him, he will start attacking you. He has melee attacks since he looks like a black knight. However, he will come together with advocates that have magical attacks.

After killing him, see what he drops. It can be anything.