Changeling

Changeling is a 1980 fantasy novel by Roger Zelazny. It was nominated for a Locus Award in 1981 & was followed by a sequel, Madwand.
The people had long suffered under Det Morson's power. When at last, the wizard Mor joined the fight, Det & his infamous Rondoval castle were destroyed. But the victory wasn't complete, for the conquerors found a baby amidst the rubble: Det's son, Pol. Unwilling to kill the child, Mor took him to a parallel world where technology ruled & the ways of magic were considered mere legends. He substituted Pol for a baby of the same age, using a spell to persuade the parents to recognize him as their own. In order to retain the balance between the worlds, Mor took the baby from the other world & brought it back to his own, leaving it with a local artisan, Marak.