Muramasa
Muramasa Sengo was a swordsmith in 16th century japan. Though an excellent swordsmith, Murasama was said to be ill-tempered, verging on absolutely insane. These began 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 Tokugaya Ieyasu allegedly (but probably not actually) banned Muramasa blades because of how many friends and family were killed or injured by Muramasa blades.
In fiction, Muramasa will usually be a rival of Masamune (Never mind the fact that they didn't live in the same time period) as well as Exclusively Evil. There is questionable testimony that Muramasa was Masamune's student.
His swords will usually be 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, etc.) The blade 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, If a character reveals they use either of these, expect the Muramasa user to be evil (or at the very least, morally grey), and the Masamune user to be good.