Changeling — Roger Zelazny

In Changeling, the people had long suffered under Det Morson's power. When at last, the wizard Mor joined the fight, Det and his infamous Rondoval castle were destroyed. But the victory was not complete, for the conquerors found a baby amidst the rubble: Det's son, Pol. Unwilling to kill the child, Mor took him to a world where the ways of magic were considered mere legends -- a world called Earth.