Sea of Shadows (Age of Legends #1) by Kelley Armstrong

In the Forest of the Dead, where the empire’s worst criminals are exiled, twin sisters Moria and Ashyn are charged with a dangerous task. For they are the Keeper and the Seeker, and each year they must quiet the enraged souls of the damned.

Only this year, the souls will not be quieted.

Ambushed and separated by an ancient evil, the sisters’ journey to find each other sends them far from the only home they’ve ever known. Accompanied by a stubborn imperial guard and a dashing condemned thief, the girls cross a once-empty wasteland, now filled with reawakened monsters of legend, as they travel to warn the emperor. But a terrible secret awaits them at court—one that will alter the balance of their world forever.