The myth of Muramasa and Masamune
Muramasa Sengo was a swordsmith in 16th century Japan and although an excellent swordsmith, he was anywhere from bad-tempered to being nuttier than a peanut farm. This gave rise to myths that swords made by him were cursed to make the wielder to fight to the death, drink/absorb blood to empower itself or even turn on their owners and compel them to commit suicide if there was no other blood to shed. The myths were given fuel by Shogun Tokugawa Ieyasu who banned Muramasa blades because many of his friends had been killed by them and he accidentally cut himself with one.
In fiction, Muramasa will usually be a rival of Gorō Nyūdō Masamune, often credited as Japan's greatest swordsmith. This discounts the fact they didn't even live in the same time period. One of many legends 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 Masamune drifted around the blade, while the Muramasa drew leaves to the blade and make them cut themselves in half. Muramasa gloated that his sword was better as his rival's didn't cut anything. Masamune stated that Muramasa's blade was inferior since it cut indiscriminately.
Of a character who uses either of these blades, the stereotype goes that the Muramasa user is evil/chaotic/rebellious, and the Masamune user is good/loyal/lawful. This is reflected in the wielders of the loyalist and rebel factions of Aslona.