Demon Blade: The Assassin Katana of Onimaro.

Once praised as "Yotsuya Masamune," the renowned swordsmith Kiyomaro was living in seclusion in the mountains with his apprentice Onimaro. However, Kiyomaro, who had begun to suffer from trembling hands due to alcohol and could no longer forge satisfactory swords, one day confided in Onimaro. Thirteen years ago, Kiyomaro had a secret relationship with a girl who served in the Ōoku (the shogun's harem), but after she caught the shogun’s attention, she took her own life. Her father, the leader of the Iga undercover agents (Isao Natsuyagi), blamed Kiyomaro for her death and sent assassins after him, forcing Kiyomaro to flee. Along the way, regretting that he had made many cheap swords just to earn some travel money, he asked Onimaro to find and discard these worthless blades. When an assassin suddenly appeared, Kiyomaro stopped Onimaro from pursuing him, but that night, Kiyomaro committed suicide.