One piece: The Greatest Swordsman of the Rocks Era!

After transmigrating into the world of One Piece, Ares joins the infamous Rocks Pirates. There, he steals Kaido’s Devil Fruit the Fish-Fish Fruit in its Azure Dragon form and embarks on a terrifying, madness-ridden path that strikes fear into every corner of the world. His name is Ares, universally recognized as the most powerful swordmaster in centuries! Rocks: “He’s the only pirate the Navy doesn’t even want to catch because capturing him would require mobilizing the entire Navy!” “And by ‘entire,’ I mean every last Marine!” Mihawk (“Hawk-Eyes”): “Even these eyes called ‘Hawk-Eyes’ can no longer discern the gap between him and me!” Linlin: “He’s not human, I've never seen a madman like him!” Kaido: “He can cut through anything in the world he desires… even my own body!” Ares is a “madman” whose mind teeters on the brink of sanity. Ares himself asks, “Tell me how can I forge myself into an indestructible, legendary blade?” Synopsis Ares awakens in the One Piece universe with memories of a world-traverser but none of the usual “system” crutches. Determined to carve out his own legend, he infiltrates the feared Rocks Pirates once the most formidable crew the sea has ever known. There, he confronts Kaido and miraculously claims the Azure Dragon form of the Fish-Fish Devil Fruit, forever altering his fate. No longer bound by human limitations, Ares channels his newfound power into mastering the sword. His unprecedented strength and ruthlessness earn him notoriety: the Navy fears even attempting his capture, Yonko tremble at his approach, and the greatest swordsman, “Hawk-Eyes” Mihawk, acknowledges Ares as his superior. Yet, beneath the legend lies a man obsessed with perfection. Haunted by a singular goal to become a blade that can cleave through any obstacle, Ares tests his limits in duels against the world’s strongest, challenges Marine admirals, and even pushes Kaido to the brink. His path of “madness” isn’t mindless chaos, but a relentless pursuit of transcendence: forging his very soul into an unstoppable weapon. As alliances form and enemies mount, Ares must decide whether his insatiable hunger for power will save him or drive him over the edge into true madness. — Description from novelcodex.org