Deathless #2: The Merciless Ones book by Namina Forna

It's been six months since Deka freed the goddesses in the ancient kingdom of Otera and discovered who she really is... but war is waging across the kingdom, and the real battle has only just begun. For there is a dark force growing in Otera—a merciless power that Deka and her army must stop.

Yet hidden secrets threaten to destroy everything Deka has known. And with her own gifts changing, Deka must discover if she holds the key to saving Otera... or if she might be its greatest threat.

The Merciless Ones is the second thrilling installment of the epic fantasy series in which a young heroine fights against a world that would dare tame her.