One Piece: Starting with Breathing Techniques

As the master of all the breathing techniques in Demon Slayer, Kiro is known as the strongest swordsman in the Demon Slayer Corps in nearly a hundred years. Just after finally slaying Muzan Kibutsuji, he unexpectedly transmigrated to the world of One Piece, becoming a civilian about to be bombarded by warships during the Ohara incident. Fortunately, his cheat ability didn't disappear; it came with him to this world. Defeating powerful enemies earns points, which can be used to upgrade skill proficiency or deduce stronger skills. Facing dire straits, Kiro remains unfazed, grinning. "The Four Emperors, the Seven Warlords of the Sea? I wonder how many of my swords they can withstand!"... During the Marineford War, the roar of [Thunderbolt Flash - Thunder God] resounded across the sky, black and red lightning leveling Marineford, the Marine Headquarters. In the New World, the Marked Swordsman's figure is like a demon god, his longsword's flames tearing through the sky, clouds churning endlessly. On Egghead Island, the scorching sun's flames burn fiercely, a dazzling red sword finally annihilating all sins. ...Many years later. Mary Geoise, a sacred land that had stood for centuries, crumbled in the flames of war. Kiro, now the world's greatest swordsman, stood atop the heavens, facing a horde of terrified onlookers. "Are you insane?! How dare you slaughter the noble Celestial Dragons?!" "You're making an enemy of the entire world!" Kiro roared with laughter. "That wretched demon, disguising himself as a Celestial Dragon! As a former Demon Slayer, I can't stand idly by!" "And besides, you've got something wrong—" "It's not me making an enemy of the world, it's the world making an enemy of me!"