Beneath The Abyss: Mermaid Queen

Mermaid queen Lorna sacrifices a thousand years of power to save Cyrus, only to see him credit his fiancee for the rescue. She gives up her tail and voice to enter his world, then awakens a darker bloodline when betrayal nearly destroys her. Returning as a queen, Lorna hunts the secret order responsible for her people's slaughter and ultimately leads the sea clans to freedom.