"Three Against the Witch World" (1965) begins a very fine trilogy of the adventures of Kemoc, Kyllan, and Kaththea, the three children of Earth-born warrior, Simon Tregarth and his witch-wife, Jaelithe. The following two books are "Warlock of the Witch World" (1967) and "Sorceress of the Witch World" (1968).
Kaththea Tregarth, born one of three triplets, could link telepathically with her birth-brothers, Kyllan and Kemoc. At an early age, she was forcibly separated from her brothers and taken to the Place of Silence to be trained in magic by Wise Women of Estcarp.
In "Three Against the Witch World" the triplets escape from witch-ruled Estcarp to the magical land of Escore. There, they accidentally destroy the false peace that had long abided between the great powers of Light and Dark. "Things awoke and gathered, and the land was troubled..." and the three learn that they must fight with the forces of Light, or be utterly annihilated by the Dark.
Kyllan, the warrior brother narrates this first book of the trilogy. After he and his brother free their sister, Kathea from the Witches, they make the long, arduous journey over-mountain to the forbidden lands of the East. Then Kaththea troubles the land by sending forth a familiar, and Kyllan is magicked and carried away by a creature of the dark in the form of a black stallion. He almost perishes but for the healing powers of the Lady of Green Silences. Once he is reunited with his brother and sister, they learn how Kaththea's witchcraft disturbed the magical balance of ancient Escore. Kyllan returns to Estcarp to recruit new soldiers for the battle to come.
Andre Norton was a powerful mythmaker and world-builder (she passed away on 03/17/2005 at age 93), and her fantasies concerning the triplets Kemoc, Kaththea, and Kyllan do not suffer in comparison with Ursula K. Le Guin's Earthsea books. In fact, my own personal preference is for Norton's Witch World.