The Priory of the Orange Tree by Samantha Shannon

The House of Berethnet has ruled Inys for a thousand years. Still unwed, Queen Sabran the Ninth must conceive a daughter to protect her realm from destruction—but assassins are getting closer to her door.

Ead Duryan is an outsider at court. Though she has risen to the position of lady-in-waiting, she is loyal to a hidden society of mages. Ead keeps a watchful eye on Sabran, secretly protecting her with forbidden magic.