"The Power That Preserves" by Stephen R. Donaldson, Thomas Covenant the Unbeliever Book Three

Twice before Thomas Covenant had been summoned to the strange other-world where magic worked. Twice he had been forced to join with the Lords of Revelstone in their war against Lord Foul, the ancient enemy of the Land. Now he is back -- to a Land ravaged by the armies of Lord Foul. The Lords are besieged and helpless. No place is safe and Foul's victory seems certain. Only Covenant can avert it. Desperately and without hope, he sets out to confront the might of the Enemy. With him go a Giant, a Bloodguard and the madwoman he has wronged. And in Foul's Creche, Lord Foul grows in power with each new defeat for the Land.