Executioner Mercurius

Description
Executioner Mercurius is the final boss in Fortress of Sorrows.

Strategy
Mercurius spawns Fallen Legion soldiers, which must be defeated before he becomes attackable. Be wary of Hell Fire cast is extremely high damage ability.

Loot
Tempering Stones Oblivion Bone Tyrant recipes 

Additional Information
If player has Quest Ma'art of the Fallen Legion Mercurius will drop an inert lair keystone, in all difficulties..