Abaddon

Archangel of the Abyss and also known as "The Destroyer"
In essence a Pit Fiend, but each of these has his own own character and strengths
Is known to have slain an Elven King and a Titan, stealing from the Titan his "mace of Hellfire"
Abaddon was then imprisoned by the Gods after misdeads in a place where he could not escape, along with some of his most loyal and deadly captains.
This place has now been located by Sarn Durgba who has made a pact with Gorthir, Half Troll Half Demon, wields famous magical Hammer of the Underworld , that he break Abaddon out to assist him in getting his revenge.
This place has been found to be in the Halls of Gold  in the nameless hills.
Gorthir, Half Troll Half Demon, wields famous magical Hammer of the Underworld  has bribed Eorg the dwarven priest of Vulcan - Flamen , to help him carry out an evil mission to break the prisons and summon Moloch 
However the party has interrupted the attempt to bend Abaddon  to their will, though he is still trapped in his cell. 
Abaddon has his own plans and agenda, and following the mad elf Sarn Durgba  or an abomination between a Troll and a Balor are not high on this agenda
Revenge pain and Tyranny may rate slightly higher....