Duke Ex Threw Me Away, His Uncle Made Me His Queen

She is half mermaid, born with scales that glow faintly under her skin, and her family sells her into marriage to settle a debt. The man who takes her as wife is a duke who treats the marriage as an inconvenience and treats her scales as something to be hidden or mocked in front of guests. Servants in his household avoid touching her plates and cups. When the duke falls into danger, a wound that will not close or a poison that ordinary medicine cannot draw out, she gives part of her own body to save him, drawing on what her mermaid blood can do that human blood cannot. The cost to her is real and lasting. He recovers, and instead of gratitude he keeps his distance from her, unwilling to be seen owing his life to a wife he never wanted. She is left weakened, half hidden in the house, still scorned for the same scales that saved him. A prince passing through the household notices what the duke's people have overlooked, that she is not sick from some ordinary ailment but worn down from having given up part of herself for a man who will not acknowledge it. He starts asking questions the servants will not answer directly, and eventually pieces together what she did and what it cost her. He removes her from the duke's house. Once she is away from that household and recovering under his protection, she has to decide whether to go back to the duke, who now understands what she sacrificed, or to stay with the prince, who found out the truth without being told and treated her differently once he knew it.