Diablo, Lord of Terror, is the Dungeon Boss for Hell.
For other fights with diablo, see Diablo
Diablo has constant immolation and will cast an explosion with a huge radius which can decimate summons. He casts rain of fire which will summon several Diablos Infernals. He will occasionally cast fireball and stun the hero.
About halfway though the battle he will transform, gaining the Demon Sword of Diablo increases his damage by 7,500. At this point, he will start summoning twice as many Infernals, and Acolytes will appear in the corners and head towards him. If they reach him, they will heal him slightly.