I Play the Devil in One Piece

([Slow Burn] + [Early development in all directions, joining a crew later] + [Prototype] + [Acquiring other people's Devil Fruit abilities, with limits])

After transmigrating into Pirate King, Mercer came with an unknown Devil Fruit. He was kindhearted and had planned to build up enough strength to lie flat and take it easy, but after one loss of control, he had no choice but to sink into the abyss...

Zephyr: Mercer, that kid is a lot more ruthless than Sakazuki.

Golden Lion: Just be a good boy and become my weapon, Mercer. Let's conquer this world together!

"Revolutionary" Dragon: Comrade, I'm begging you. You have to join our revolution.