Ensii by Elisabeth Wheatley

The stunning conclusion!
Talitha expected a fight to take back Ilios, but she returns to her city to find her enemies have gone underground. Worse, the deposed usurper has invoked powers beyond the realms of flesh and blood.
The priests of the war goddess have unleashed something terrible, something wicked. Talitha and Ashek must find a way to destroy it before the monstrosity consumes their reclaimed city and everything they’ve fought for.