Lokarr

Lokarr, Master of the Crucible is both an NPC and a Celestial Super Boss. He is the Facilitator of the Crucible, and if you follow the quest Lokarr's Secret Quest you will fight him as the boss in the Crucible of the Dead.

Tips for fighting him
During the fight he will summon Clones of John Bourbon.

Most of his damage is fire so going in with over capped fire resistance will help tremendously.