By the Sword(1991)

Mercedes Lackey

Granddaughter of the sorceress Kethry, daughter of a noble house, Kerowyn had been forced to run the family keep since her mother’s untimely death. Yet now at last her brother was preparing to wed, and when his bride became the lady of the keep, Kerowyn could return to her true enjoyments—training horses and hunting.

But all Kerowyn’s hopes and plans were shattered when her ancestral home was attacked, her father slain, her brother wounded, and his fiancée kidnapped. Drive by desperation and the knowledge that a sorcerer had led the attack, Kerowyn sought her grandmother Kethry’s aid, a journey which would prove but the first step on the road to the fulfillment of her destiny. For facing her family’s foes would transform Kerowyn into an outsider in her own land, a warrior bound to the spell blade Need, and a mercenary forced to choose between loyalty to her comrades in arms and the Herald of Valdemar, whom she had rescued and who in his turn had helped to awaken her to the true meaning of love and to her own unique powers of magic.