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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[by whom?] 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.
The dates of Muramasa being active post 1500 are supported by the first dated work in 1501. It was not unheard of for some Nihonto smiths to live 100 years and a smith could feasibly have had an active period of 80 years, working with assistants towards the end of their life. Swords attributed to the name Muramasa may not belong to a single individual. There may have been three generations working before the one who signed the 1501 blade. Muramasa blades were recorded to be in the hands of famous Generals and Daimyo by 1535.
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.
Much like his unique reputation, Muramasa is known for some fairly unique features in his work. The first particular characteristic of his is the frequent use of a mirror image hamon. The hamon of Muramasa is usually midareba with very shallow valleys (almost touching the ha) between a cluster of gunome shapes. The other easily identifiable feature one will see on Muramasa blades is the fish-belly (tanagobara) shape of the nakago.
"A story is told of an incident in Edo Castle. On Bunsei 6th (1823), Matsudaira Geki is said to have killed three men against whom he long held a grudge and the sword Geki used was a Muramasa. Geki was working with 5 others in the library of Nishimaru, he suddenly got up and without a word swung his sword, at which the head of Honda Iori is supposed to have flown off at the shoulders. Toda Hikonoshin got up to run but Geki cut him down diagonally in one strike across the shoulder. Numata Sakyo was slashed across the waist and the second stroke down at the shoulders. The three were cut down in 4 strokes. The others, Kami Goro and Mabe Genjuro tried to run but also were cut down the back and across the buttons, though the last two did not lose their lives. This will indicate the capabilities of the Muramasa blade in the hand of a fairly good swordsman." -Excerpt from Albert Yamanaka - JSS-US article
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.
In Popular Culture
- In the RPG Golden Sun, there is an end-game sword named Muramasa which can be obtained by the player. Keeping true to the legends about Muramasa's swords being evil, this blade is a cursed weapon, and has a special attack called "Demon Fire" in which it engulfs a target in hellish flames and proceeds to slash them head-on. However, the blade bears no resemblance to any Japanese sword, instead resembling a European longsword.
- In Marvel Comics, two swords made by Muramasa have appeared, along with the apparently immortal Muramasa. The first, wielded by Wolverine, then the Silver Samurai seemed to be cursed to be intrinsically malevolent and could only be safely wielded by those with a particular strong will, much like the Ebony Blade. The second wasn't cursed, but carried a portion of Wolverine's soul, being one of the very few artifacts that could reliably kill him.
- In the game Terraria, there is a sword named after Muramasa. It has a high attack speed, to represent cutting power, but is not particularly strong. In early versions of the game, it was considered the most powerful weapon until later patches reduced its power significantly.
- In the game Grand Chase also has a sword that makes reference and which is used by Uno, character where the character is mad for blood and is not satisfied. It also has a special skill that your body is dominated by something demonic cutting out everything before that is also reference to the game Golden Sun.
- In the anime Bleach, there is a sword named after Muramasa. He is the main character of the Muramasa and Tōjū filler arcs. 
- Ratti, Oscar and Adele Westbrook (1991). Secrets of the Samurai: The Martial Arts of Feudal Japan. Tuttle Publishing. p. 263. ISBN 0-8048-1684-0.
- Sengo Muramasa school lineage
-  www.Muramasa.us/history
-  www.Muramasa.us/features
- Stone, George Cameron (1999). A Glossary of the Construction, Decoration, and Use of Arms and Armor in All Countries and in All Times. Dover Publications, Inc. p. 460. ISBN 0-486-40726-8.