Rise, Queen of the Deep

Diana, the mermaid princess, flees from the forced marriage to the obsessive Sea King Triton. She conceals her identity, disguising herself as a fragile veiled maid, and wanders into the human world where she is taken in by the kind Duke Sebastian. The duke's domain is plagued by assassinations and conspiracies over its rare mineral resources. Diana resolves to protect him in secret, but dares not use high-level ocean magic, fearing that her magical aura will be tracked by Triton's artifacts and bring catastrophe upon them. In her darkest hour, she awakens the divine power of the Deep Sea Queen—commanding tides and sea monsters—defeating and banishing Triton.