The Coldfire Trilogy(1991)

More than a thousand years ago, humanity colonized Erna, a distant planet inhabited by a mysterious force known as the Fae, which gives physical form to human desires and nightmares. After centuries of struggle, humanity has learned to survive in this dangerous world, but the Fae remains a constant threat, and its power is increasingly exploited by those seeking to control it. At the center of the story are Damien Vryce, a warrior-priest devoted to protecting humanity, and Gerald Tarrant, an immortal sorcerer bound to the darkness he once embraced. Forced into an uneasy alliance in the face of a growing threat, they embark on a perilous journey that will test their faith, ambitions, and very understanding of good and evil.