Sword of Kusanagi
The Sword of Kusanagi (草薙の剣 Kusanagi-no-Tsurugi, lit. "Grass Cutting Sword"), but originally called Ame-no-Murakumo-no-Tsurugi (天叢雲剣, lit. "Sword of the Gathering of Clouds in Heaven"), is a legendary artifact and one of the "Three Sacred Treasures of Japan". According to legend, it was first discovered when the storm god Susanoo was chopping off the fourth tail of the fearsome eight-headed, eight-tailed serpent Yamata-no-Orochi while it still asleep from drinking extremely heavy sake. It got the name "Grass cutting sword" when the great warrior Yamato Takeru was trapped in an open grassfield by an treacherous warlord with fiery arrows and had to use the sword to cut back the flames by removing some of its fuel. It is often mistaken to be a katana because it is a Japanese weapon, despite the Kusanagi pre-dating the invention of the katana by a number of centuries.
Kusanagi in Len'en
Despite it seldom being referred to as "Kusanagi", it still makes an appearance and it plays a major role in Earthen Miraculous Sword, called the Tasouken. Right now, the sword is in the possession of Adagumo no Yaorochi, a youkai born from the severed arm of a clan leader (implied to be Yamata no Orochi itself). The essence of the sword was transferred into a clay replica (later revealed to be a sword-shaped bottle opener) by Sukune Katano as requested by Adagumo no Saragimaru, a youkai born from the clan leader's eye and is the younger sibling of Yaorochi.