Muramasa Sengo (千子 村正 Sengo Muramasa) was a famous swordsmith who founded the Muramasa school and lived during the Muromachi period (14th to 16th centuries) in Japan. Oscar Ratti and Adele Westbrook said that Muramasa "was a most skillful smith but a violent and ill-balanced mind verging on madness, that was supposed to have passed into his blades. ... They were popularly believed to hunger for blood and to impel their warrior to commit murder or suicide."
The school of sword-making at Ise province was famous for the extraordinary sharpness of their blades. The earliest known work of the school is dated at 1501; the Muramasa school continued into the late 16th century. It is believed that Sengo Muramasa was a student of Heianjo Nagayoshi, a prominent Kyoto swordsmith known for spears and engravings. Other legends hold that he was a student of Masamune, which is impossible, as Muramasa lived much later than Masamune.
Muramasa's swords fell out of favor with the Japanese government when Tokugawa Ieyasu became shogun, establishing the Tokugawa Shogunate, in 1603. It is said that Ieyasu had lost many friends and relatives to Muramasa blades and had cut himself badly with one, so he forbade his samurai to wield blades made by Muramasa. This contributed even more to the Muramasa legend and led to many plays and dramas in Japanese literature featuring the blades. Due to the stigma attached to them, many Muramasa blades had their signature changed or removed. Since opponents of the Tokugawa Shoguns would often wish to acquire Muramasa blades, forgeries of Muramasa blades were also often made.
The swords of Muramasa are often anecdotally contrasted with those of Masamune, another Japanese swordsmith who lived some 300 years earlier, around 1264–1343 AD.
There is a legend of a Masamune blade and a Muramasa blade being put into a river strewn with lotus leaves. The leaves swirled around the Masamune blade untouched, but the Muramasa blade would cut them.
It has also been told that once drawn, a Muramasa blade has to draw blood before it can be returned to its scabbard, even to the point of forcing its wielder to wound himself or commit suicide. Thus, it is thought of as a demonic cursed blade that creates bloodlust in those who wield it.