Mirror Bound by Rhiannon Held

Verity is a phantom, a being of the mirror realm who can change her face—or she could before she was bound by a mage of the grounded realm. Now she’s trapped in a human form, forced to tell the truth and obey the one who holds her binding: Dakota, a hunter who, with her team of other mages, keeps the world safe from the more monstrous creatures of the mirror realm. But Verity’s binding is weakening. Every time she teleports the team to safety, she gets a little closer to freedom.Except the number of monsters coming from the mirror realm is increasing and the mages’ governing body, the Quorum, can’t figure out why. With monsters killing humans and all eyes on Verity, she’s fast running out of time to free herself—especially when she discovers that unless she can stop them from gaining access to the mirror realm, the Quorum’s plans for her realm and her people are far more dangerous than anything they might do to her.