Magic's Price — Mercedes Lackey

VALDEMAR --
the once-peaceful kingdom protected by the magic of its Herald-Mages--is now beseiged on all fronts. The king lies near death, the neighboring land of Karse wages a relentless war against Valdemar, and the forces led by a master of dark forbidden magic are massing to strike the final devastating blow against the kingdom.

And Vanvel, the most powerful HeraldMage Valdemar has ever known, has become the primary target of the evil which is reaching out to poison all the land. With all his fellow mages slain, Vanvel alone remains to defend his people against the dark master's arms. Yet a dream vision has revealed to Vanvel the fate which awaits should he and his Companion Yfandes take up the dark master's challenge. And if either Vanvel or Yfandes falters, the dream will become a horrifying reality in which both Valdemar and its last Herald-Mage must pay the ultimate price....