The Merfolk King is my exclusive loyal dog

Betrayed and drained to death by my merman king Dante and cruel elder sister, I reborn at a mermaid auction. I dump the ungrateful Dante and pick Milo, a powerful merman loyal only to me. I trick Dante to avenge my past suffering, grow close with Milo, humiliate my vicious sister, and fight for the merman throne at the grand tournament.