Blood Sanctuary: Part One by Vela Roth — book slip

Cassia has reunited with her beloved Lio in the land of eternal night. She longs to become an immortal Hesperine like him so they can stay together forever. But she's torn between their happiness and her duty to the kingdom her human father rules. To free herself from the king, she must convince his ambassadors to ally with the Hesperines, despite centuries of conflict.

Lio will die if Cassia leaves him, but he won't force her to stay by revealing their blood bond. While they negotiate for peace in public, he tempts her in private with the promise of everlasting passion. But their future may end before it begins when enemies within the embassy threaten her with a lethal spell.