The Stone Key (The Obernewtyn Chronicles #6)

Friend becomes foe and trust is a thing of the past when the Misfits’ most relentless enemy turns Elspeth’s world upside down. Through coercion and mind control, Ariel stands on the cusp of his ultimate revenge—and it falls to Elspeth to rescue a former ally whose mind and body have been manipulated to unleash a plague that will destroy all it touches.