A Realm Divided — Juliet Anderson

Erin is dreading her eighteenth birthday when she inherits her full powers; she is terrified she'll turn dark and evil like Heinrekh. She has plenty of other complications to focus on too. Her closest friend is dating her arch enemy. She is exiled from Lokranor along with her guardian Magnus and Prince Wilhelm. And her new tutor is one of Valkarin's loyal guards. As she digs deeper into the secrets her house holds, she uncovers a host of unexpected alliances, betrayals and hidden books. Rescuing her mother is always at the front of her mind, and that chance arrives at last. But is she truly strong enough to beat Heinrekh at his own game? He finds out that teenagers rarely do what is expected of them and can play just as dirty. Now if only she can keep her mind off a rather cute Viking who has just moved into her house, everything would run much more smoothly.
Book 2 of the Realms Gate Series.