Crown of Shadows: The Coldfire Trilogy, Book Three

Synopsis: For more than a millennium now Erna’s humans have maintained an uneasy stalemate with the fae, that treacherous force of nature which feeds on the human psyche.  Adepts and sorcerers work the fae for their own profit, while the demonic creatures who feed upon such efforts rapidly gain in power and ambition. Now one of these demons, a Iezu called Calesta, has declared war on all of mankind.
 
Master of illusion, devourer of pain, he plans to remake the human species until mankind exists only to sate his unquenchable thirst for suffering, and omens of his triumph are already apparent.
 
Only Damien Vryce, warrior-priest of the One God, and his unlikely ally, the undead sorcerer Gerald Tararnt stand between Calesta and his triumph.  Nothing short of the demon’s absolute destruction will save mankind from his unholy influence. But no one on Erna is certain just what the Iezu really are and no man has ever succeeded in killing one.
 
Faced with an enemy who may prove invulnerable, Damien and Tarrant must risk everything in a war that will take them from the depths of Hell to the birthplace of demons and beyond—in a battle which could cost them not only their lives, but the very soul of all mankind.