Was originally owned by Miyoshi Masanaga, whose priest name is Miyoshi Sousan (where the sword's name is derived from). The sword would later be given to Takeda Nobutora as a gift.
At 1537, Nobutora's daughter Joukeiin married Imagawa Yoshimoto in order to strengthen ties between the Takeda and Imagawa, and the sword was given as part of her dowry, therefore the sword is also known as Yoshimoto Samonji.
After Yoshimoto's death during the Battle of Okehazama (1560), Oda Nobunaga took possession of the sword, shortened the blade, and engraved on sword's tang (the section of the blade that is placed into the hilt), “On the 19th day of the fifth month of Eiroku three through the killing of Yoshimoto this sword came into the possession of Oda Owari-no-kami Nobunaga ”.
Later, Nobunaga died during the Honnoji Incident (1582), and the sword came into possession of Toyotomi Hideyoshi, one of Nobunaga's retainers and later the Second Unifier of Japan.
After Hideyoshi died of illness at 1589, there was a power struggle between Ishida Mitsunari (on behalf of Hideyoshi's infant heir) and Tokugawa Ieyasu (on behalf of himself), which led to the Battle of Sekigahara (1600), where the Tokugawa forces won. The Samonji sword would later be given to Ieyasu, and passed over the Tokugawa clan.
The sword was burnt during the Great Fire of Meireki at 1657 and had to be reforged. It’s said that the new hamon (the pattern of the steel on the edge of the blade) was far inferior from the original, but nonetheless, the blade survived.
Eventually Emperor Meiji decided to open several shrines commemorating Nobunaga’s achievements, and the Samonji sword was donated to one of such exhibits, where it remains today.
He was once owned by all of the 3 Unifiers of Japan (Nobunaga, Hideyoshi, and Ieyasu), thus he's regarded as the sword of conquerors.
Despite his powerful owners, he was treated more like a possession rather than a battle sword, which greatly affects his personality (especially during Nobunaga's possession, where he was traumatized by the mark engraved upon him).
His mark has the kanji 第六天魔王 (Dairoku Tenmaou, Demon King of the Sixth Heaven) written over it. The phrase refers to Nobunaga's title.