A sword that is part of the legal apparatus of a modern constitutional state, that must be present at the accession of a head of government, and that no living person is permitted to look at — including the man it is handed to. Whether the object that exists is the object the stories describe has been an open question since the twelfth century, and there is no procedure by which anyone could find out.
Two names, and the change between them
It comes out of the fourth tail of the eight-forked serpent, where Susanoo's blade chips on it, and its first name is Ame-no-Murakumo-no-Tsurugi, the sword of the gathering clouds of heaven — the Nihon Shoki explains that clouds always hung over the serpent, which is a rare instance of the chronicles glossing their own naming.
It becomes Kusanagi, grass-cutting, several generations later, and the renaming is the whole of its second story.
Yamato Takeru, sent east by his father to subdue the provinces, receives the sword from his aunt Yamatohime, the priestess at Ise, along with a bag she tells him to open only in an emergency. In Suruga a provincial chief invites him to hunt on a moor and then sets fire to the grass around him. He cuts the grass down with the sword, opens the bag, finds a fire-striker, and lights a counter-fire that burns outward to meet the advancing one. He survives; the men who set the trap do not.
The Kojiki gives the sword the credit for the cutting, hence the name. The Nihon Shoki offers a variant in which the grass mows itself, and another explanation of the name entirely. The place is Yaizu in Shizuoka, whose name is generally taken to preserve the burning.
Yamato Takeru then leaves the sword with his wife at Owari before going to fight a deity on Mount Ibuki bare-handed. He is defeated by it, sickens, and dies on the road home. Leaving the sword behind is presented as the mistake that kills him, and it is the only occasion in Japanese myth where a hero's fate turns on where he put a weapon.
Atsuta, and the theft
The sword stayed in Owari, and Atsuta Jingū in Nagoya was founded to house it. Its custody there is asserted continuously from the eighth century.
It has left twice, in accounts the shrine and the chronicles both preserve. In 668 a Korean monk named Dōgyō is recorded as having stolen it and attempted to carry it out of the country; contrary winds drove his ship back and it was recovered. It was then held at the imperial palace, until the illness of Emperor Tenmu in 686 was attributed to its presence and it was returned to Atsuta. A sword whose proximity is diagnosed as the cause of a sovereign's sickness is not being treated as a decorative heirloom.
Dan-no-ura, 1185
At the sea battle that ended the Genpei War, the Taira fleet was destroyed in the straits at Dan-no-ura. The child emperor Antoku, aged six, was on board with the three regalia. His grandmother took him in her arms, told him there was a capital beneath the waves, and went over the side with him.
The jewel was recovered floating in its casket. The mirror was recovered. The sword was not.
The Heike Monogatari reports this with an explanation that is worth reading as a piece of theology rather than of history: the dragon king of the sea, it suggests, had lent the sword to the imperial house through the serpent's tail, and had now taken it back. A weapon that entered the human world out of a water-monster returns to the water.
What has been at Atsuta ever since is therefore either the original — on the argument that a replica went down at Dan-no-ura and the shrine's sword was never at sea — or a replacement made afterwards, or the same replica the palace had been using since the seventh century. Medieval commentators argued all three positions and the argument has never closed.
Seen, once, by someone who should not have
The shrine record most often cited is a seventeenth-century account, transmitted from Atsuta, in which several priests broke open the casings and looked. The description given is of a blade of moderate length with a whitish metal colour and a shape thickening towards the tip. Every priest involved is said to have suffered afterwards, and several died.
The document is late, transmitted at second hand, and impossible to verify. It is repeated in almost everything written about the sword, and it should be repeated with that caveat attached: it is the only physical description anyone claims, and it is a piece of shrine lore about the consequences of looking.
Present, unseen, in 2019
At the enthronement of the present emperor in May 2019, the ceremony of the inheritance of the sword and the jewel was broadcast. Two attendants carried in wrapped bundles and set them on tables, along with the state and privy seals. The bundles are the sword — a substitute kept at the palace, since the Atsuta blade does not travel — and the jewel.
The mirror does not appear at all; it remains at Ise, with its own palace replica in the imperial sanctuary. The ceremony took under ten minutes, no object was unwrapped, and only male members of the imperial family attended, which drew criticism at the time.
The regalia's constitutional position is genuinely unresolved. They are not state property in the ordinary sense, they are not subject to cultural-property designation, and no government official inspects them. A country with an exhaustively documented legal system contains three objects that are outside its ordinary categories entirely, and the mirror is the most inaccessible of the three.
Every sword in every game
It is probably the single most reused weapon name in Japanese media. Final Fantasy, Ōkami, Nioh, Sekiro-adjacent titles, Naruto, Yu-Gi-Oh!, Fate, Smite and an enormous number of gacha games field a Kusanagi or an Ame-no-Murakumo, generally as a wind- or storm-attribute blade. The King of Fighters named a protagonist Kyo Kusanagi and gave his family the mythological pedigree explicitly.
What almost never appears is the grass. A sword named for cutting vegetation to escape a fire is a strange thing to build a legend on, and modern versions consistently reassign it to clouds, storms or the serpent — which is to say they prefer the first name, and the story attached to the second is the one the Kojiki actually thought was important.
Could the sword be examined scientifically?
There is no legal or institutional route to it, and no serious proposal has ever been made. The Imperial Household Agency does not open the regalia, does not permit inspection of imperial tombs either — a long-running frustration for Japanese archaeology, since several of the largest keyhole tombs in the country are designated as imperial graves and closed — and treats the objects as sacred rather than as heritage. The comparison with other traditions is instructive: a European crown is displayed in a museum case and dated by metallurgy, while a Japanese regalia object houses a deity rather than commemorating one, and looking at it is not an examination but a violation. What can be studied is everything around it — the shrine records, the Heian and medieval commentaries, the archaeology of comparable iron blades — and that work is active.
What kind of sword would it actually have been?
If any physical object underlies the story, the plausible candidates are the straight, double-edged or single-edged iron blades of the Kofun period, of continental type, which are found in Japanese tomb mounds in quantity — some of them inscribed, one famously with a date and a king's name. The curved katana that everyone pictures is a much later development, emerging around the tenth to twelfth centuries, and no sword of that shape could belong to the period the myth describes. The seventeenth-century description transmitted from Atsuta, for what it is worth, does not describe a curved blade. Illustrations of the regalia in popular media almost invariably show a katana, which is an anachronism of six hundred years or more, and one of the more revealing anachronisms in the whole regalia tradition.