The stone key (The Obernewtyn Chronicles #5)

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.From the Paperback edition.