Warlock of the Witch World (The Estcarp Cycle #4) by Andre Norton PB Ace 87319 1967

Classic Fantasy Series. "Kyllan the warrior, Kaththea the untried witch, Kemoc, whose powers could surpass all others- these are the half-Earthling, half witch-brood family menaced by the sorceries of an unknown enemy. The burden of the struggle fell to Kemoc, who was forced to summon his untested powers in the battle to match the alien evil threatening the Witch World."

Cover art by Jack Gaughan