Muramasa Sengo was a swordsmith in 16th century Japan. Though an excellent swordsmith, he was said to be insanely ill-tempered, verging on just straight-up insane. This trait led to myths that swords made by him were cursed with the same afflictions, encouraging the wielder to fight to the death, and would, if no battle was present, turn on their owners and compel them to commit suicide. These rumors weren't helped by the fact that Shogun Tokugawa Ieyasu banned Muramasa blades because many of his friends had been killed by them, and that he had accidentally cut himself badly with one (after all, history is Written by the Winners).
In fiction, Muramasa will usually be a rival of Gorō Nyūdō Masamune (Masamune). Never mind the fact that they didn't live in the same time period. A legend regarding both swordmakers said that they held a competition to see who was better by putting one of each of their blades in a stream. Leaves floating towards the Masamune would drift safely around the blade, while the Muramasa drew leaves to the blade, whereupon they would slice themselves in half. Muramasa bragged that this was evidence of his sword's superiority since Masamune's failed to cut anything, but Masamune stated that Muramasa's blade was inferior since it cut indiscriminately without control.
Muramasa's swords, which are often named after him, will usually be Always Chaotic Evil, Empathic, bloodthirsty - possibly even absorbing blood into the blade to strengthen itself - or encouraging the host to do so by cursing them.) and have an Obviously Evil appearance aura about it (Notches, rust, bloodstains, ect.) The blades may also take near inhuman force of will to sheathe the blade once unsheathed until blood is drawn with it. In sharp contrast to the swords made by his rival, Masamune's swords can be more reliable, but less overpowered.
Neither are to be confused with Murasame ("Autumn Shower"), another popular name for fictional katana, which is drawn from a magic sword in Nansou Satomi Hakkenden or one of the protagonists in the Noh play Matsukaze.
For examples of these swords in media see Public Domain Artifact.