The Deepest Vow

Deep-sea King Sarlin suffers the Abyss Curse, consumed by power at every blood tide, relieved only by merging with a mermaid bearing ice-glass markings. Low-born mermaid Ilowen, forced by her gambling father to repay debts, joins the royal sacrifice selection for money, then leaves. She unexpectedly bears Sarlin's child and is sold to debt traders.

Sarlin finds her via the markings, makes her queen. Stepmother Seraphina schemes against her repeatedly. Truth emerges: Ilowen is the last ancient Singer heir, the cure hidden in her mother's shell pendant. At blood moon, they resonate fully, sacrificing her voice to break the curse. Their child is born, and the ancient clan is vindicated.