One Piece: The Lunarian

Kaido gave Crow two choices: join the Beast Pirates and create an unstoppable army for him, or die, and he'll give his family a fate worse than death. Having no other choice, he joins but promises he'll make the Strongest Creature in the world pay. (Massive crossover, powerful mc, large harem, and other surprises.) If you don't like crossovers or harems, don't read the story!