kusanagi (probably a tsurugi, a type of bronze age sword which precedes the katana by centuries) is the most famous legendary sword in japanese mythology, involved in several folk stories. along with the jewel and the mirror, it was one of the three godly treasures of japan. a common misconception is that katanas magically sprung into existence in japan, utterly isolated from the mainland. the technique of folding steel came from the manufacture of the dao in china, and contact with the mainland would affect how the katana evolved through the centuries. the katana design itself was developed over hundreds of years and the katana design was a development of the tachi.