The Changeling Saga Series by 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.

Pol Detson, son of Lord Det of Rondoval, has come home. He is now a powerful sorceror of unsurpassed natural ability - in a world where the power of magic is the only kind that matters. But Pol is still an untrained talent, a 'MADWAND'. To take control of his powers, to rule in his father's place, he must survive arduous training and a fantastic initiation into the rites of sorcery. As friends, Pol has one dragon and one thief. As enemies he has the most powerful wizards of the land... morePol Detson, son of Lord Det of Rondoval, has come home. He is now a powerful sorceror of unsurpassed natural ability - in a world where the power of magic is the only kind that matters. But Pol is still an untrained talent, a 'MADWAND'. To take control of his powers, to rule in his father's place, he must survive arduous training and a fantastic initiation into the rites of sorcery. As friends, Pol has one dragon and one thief. As enemies he has the most powerful wizards of the land. And at least one of them wants him dead. less

"Formerly published in parts as Changeling and Madwand."