COLDFIRE by C. S. Friedman
The Coldfire trilogy tells a story of discovery and battle against evil on a planet where a force of nature exists that is capable of reshaping the world in response to psychic stimulus. This terrifying force, much like magic, has the power to prey upon the human mind, drawing forth a person's worst nightmare images or most treasured dreams, and indiscriminately giving them life.
This is the story of two men: one, a warrior priest ready to sacrifice anything and everything for the cause of humanity's progress; the other, a sorcerer who has survived for countless centuries by a total submission to evil. They are absolute enemies who must unite to conquer an evil greater than anything their world has ever known.