City of Snakes by Mariet Kay

Her people are safe, but an oath sealed in blood demands payment. Now, she must settle the debt.

Sybilla Wymark has one choice—sacrifice her friend to King Darvanda’s realm as collateral or go in her place. As Queen, the answer is simple. Sybilla will go. Wary of his realm’s magic, her allies question her safety, but where they see trouble, she sees opportunity.
Krait Darvanda smells death in the air. Time runs thin to claim his destiny, so he drags a headstrong Queen back to the Sahlms. The only way Krait can prevail is to fight alongside his spoiled, foulmouthed new ally. He has no choice; she holds a key to the ultimate weapon and his people’s salvation.