The Mermaid King

Human prisoner Elvin is kept by mermaid crown prince Cassius under the appearance of love but used as a living filter to absorb poison for favored Loran. Betrayed and cast into the abyss, Elvin accepts the power of lord Baal and returns three years later as its ruthless ruler to destroy the old empire. Cassius learns that Elvin—not Loran—was the donor who once saved him and submits to imprisonment in remorse, protecting Elvin through repeated attacks. When Baal attempts to possess Elvin, Cassius tears out his mermaid heart and burns his soul in battle. Elvin gives half his lifespan to save him, and they overthrow the class order and share the thrones of sea and abyss.