The Deceiver's Heart (the Traitor's Game, Book Two)

Now firmly on the side of the Coracks, sixteen-year-old Kestra Dallisor plans to use the magic Olden Blade to kill the evil Lord Endrick, but when she fails, Endrick turns her into one of his Ironheart soldiers, without any memory of her rebellion and bound to obey him, his spy within the Coracks. The rebel Simon is desperate to return her to the Kestra he loves, but he is reluctant to use the magic that might heal (or destroy) her--and neither Simon or Kestra is sure who they can trust among the rebels, because the whole rebellion is endangered by its increasingly murky politics.