See Vazruden for information seeing as how the two fight together.
Loot Note Edit
Upon killing Nazan, a Reinforced Fel Iron chest spawns to the side and the two pieces of loot from this encounter are in there.
Make sure you wait a bit after killing Nazan for the chest to spawn, and look around for it. The usual strategy putting Nazan in the center of the ring and rotating him as he breathes often has him dying over the chest, hiding it from view.