Nightspell by Leah Cypess

Here be ghosts, the maps said, and that was all.

In this haunted kingdom, ghosts linger—not just in the deepest forests or the darkest caverns, but alongside the living, as part of a twisted palace court that revels all night and sleeps through the daylight hours.

Darri's sister was trapped in this place of fear and shadows as a child. And now Darri has a chance to save her sister . . . if she agrees to a betrothal with the prince of the dead. But nothing is simple in this eerie kingdom—not her sister, who has changed beyond recognition; not her plan, which will be thrown off track almost at once; and not the undead prince, who seems more alive than anyone else.

In a court seething with the desire for vengeance, Darri holds the key to the balance between life and death. Can her warrior heart withstand the most wrenching choice of all?