Yamata no Orochi is a serpent-dragon in Japanese myth. The Orochi has eight heads and eight tails, and its enormous body reaches across eight valleys and eight hills. (You're probably noticing a pattern here.)
When Susano is in exile from the heavens, he finds a couple and their daughter crying by the river. They explain their sadness to him — that every year, the Orochi comes to devour one of their daughters. This year, they must give up their eigth and final daughter, Kusinada.
To save her, Susano proposes marriage to Kusinada. When she accepts, he transforms her into a comb which he can then carry in his hair. Kusinada's parents must brew sake, he explains, and refine it eight times. They must also build an enclosure with eight gates, each of which includes a vat of sake.
When the Orochi arrives, he is lured in towards the sake, and dips each of his heads into one of the vats. The drunken beast is now weakened and disoriented, allowing Susano to quickly slay it.
As Susano cuts the monster into pieces, he uncovers a great sword that had grown inside the Orochi. This blade, the Kusanagi, is presented to Amaterasu as a gift to reconcile their dispute.