10,000 cold nights is technically a “tachi” style blade crafted in the late 14th century. According to legend, it MAY be one of the first swords in the Muramasa school of swordsmithing.
A legend tells of a test where Muramasa challenged his master, Masamune, to see who could make a finer sword. They both worked tirelessly, and when both swords were finished, they decided to test the results. The contest was for each to suspend the blades in a small creek with the cutting edge facing against the current. Muramasa’s sword, the Juuchi Yosamu (十千夜寒, “10,000 Cold Nights”) cut everything that passed its way; fish, leaves floating down the river, the very air which blew on it. Highly impressed with his pupil’s work, Masamune lowered his sword, the Yawarakai-Te (柔らかい手, “Tender Hands”), into the current and waited patiently. Only leaves were cut. However, the fish swam right up to it, and the air hissed as it gently blew by the blade. After a while, Muramasa began to scoff at his master for his apparent lack of skill in the making of his sword. Smiling to himself, Masamune pulled up his sword, dried it, and sheathed it. All the while, Muramasa was heckling him for his sword’s inability to cut anything. A monk, who had been watching the whole ordeal, walked over and bowed low to the two sword masters. He then began to explain what he had seen.
The first of the swords was by all accounts a fine sword, however it is a blood thirsty, evil blade, as it does not discriminate as to who or what it will cut. It may just as well be cutting down butterflies as severing heads. The second was by far the finer of the two, as it does not needlessly cut that which is innocent and undeserving.
In yet another story Muramasa and Masamune were summoned to make swords for the shōgun or emperor, and the finished swords were held in a waterfall. The result is the same as the other stories, and Masamune’s swords are deemed holy swords. In one version of the story Muramasa is killed for creating evil swords.