Gorō Nyūdō Masamune was a Japanese swordsmith active during the late Kamakura period (late 1200s to early 1300s). He is widely considered the greatest swordsmith in Japanese history. His blades were celebrated for their exceptional clarity, beautiful grain patterns, and perfectly executed hamon lines. Authenticated Masamune swords are classified as Japanese National Treasures and are held only in museums and imperial collections — none have ever been publicly sold.