Winds of Change (Valdemar: Mage Winds #2)

Dark Magic

With Valdemar in dire peril-threatened by the malevolent spellcraft of Ancar of Hardorn-Princess Elspeth, Herald and heir to the throne, has come to the Vale of the Tayledras 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 tame the rouge Heartstone before the next strike of the Dark Adept, risking the perils of the unknown in a desperate bid to save both their peoples...