Winds Of Change by Mercedes Lackey Book

     None but the Companions remember the long-ago age when high magic was lost to Valdemar as the last Herald-Mage gave his life to protect his kingdom from destruction by dark sorceries. But now the protective barrier set so long ago over Valdemar is crumbling, and with the realm imperiled by the dark magic of Ancar of Hardorn, Princess Elspeth, Herald and heir to the throne, has gone on a desperate quest in search of a mentor who can teach her to wield her fledgling mage-powers and help her to defend her threatened kingdom.