Winds of Change — Mercedes Lackey

DARK MAGIC

With Voldemar in dire peril--threatened by the malevolent spellcraft of Ancor of Hardorn--Princess Elspeth, Herald and heir to the throne, has come to the
Vale of the Toyledros Clan to seek Mage training among the magical Hawkbrother Adepts. But instead of finding a haven, she is whirled into a maelstrom of war and sorcery as the Vale is attacked by a mysterious Dark Adept from out of the "Uncleansed Lands."

And when the Heartstone, the source of magical power for the Clan, is warped by evil sorceries and turned into a dangerous rogue, disabling the most powerful of the Hawkbrother Adepts, Elspeth--still only a half-trained Mage---and the renegade Hawkbrother-Adept Darkwind must struggle to tome the rogue Heartstone before the next strike of the Dark Adept, risking the perils of the unknown in a desperate bid to save both their peoples....