Duskbound (Esprithean Trilogy)

Fia Riftborne thought joining the Sidhe Guard was the hardest choice she'd ever make. She was wrong. Taken prisoner by the very shadows she was sworn to fight, Fia uncovers a devastating truth. The wraiths who brought destruction to the Isle's western border aren't monsters at all, but a race of people from a dying realm called the Kálfar, and they believe Fia's growing powers are tied to their survival. As war looms on two fronts, Fia must navigate a world of shadow magic and political upheaval. A disgraced Kálfar lord builds an army in the south while tensions within Sídhe reach a breaking point. And the Void that gave Fia her power is calling—but answering it means betraying everyone she left behind. The Guard. Her friends. Laryk. In a war where nothing is as it seems, choosing the wrong side could cost her everything, including the brooding commander whose darkness seems to recognize her own.