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Yamata-no-Orochi (八岐大蛇 or ヤマタノオロチ or やまたのおろち Yamata-no-Orochi, meaning Eight-Branched Serpent) is a serpentine yōkai or god and one of the most powerful yōkai in existence.
Yamata-no-Orochi is an eight-headed serpentine dragon with a single body, eight tails, dark blue skin, green fur on each of its heads and coverings of moss on it's body.
It is both a god of river or mountains, or a monster of an incarnation of floods. Although Medama-Oyaji noted that it is different from dragons, Yamata-no-Orochi has often been regarded as a type of dragon or dragon god, and the classification between dragons and reptilian beings have been rather vague in Japanese folklore and in the series.
He appeared in the sixth anime #73 episode, where he was trapped inside a gem. He was able to grant eight wishes to the wielder of the gem, which would in turn create misery, after the eight wish, the wielder would turned into a Yobuko.
The Yamata-no-Orochi's name has several different letters and meanings; 八岐大蛇, 八俣遠呂智, 八俣遠呂知, 高志之八俣遠呂知; Eight-Headed/Branched/Immensely Huge/Mountainous - Serpent/Spiritually-Powerful Being/Long-Tailed God of Koshi Province.
The Yamata-no-Orochi terrorized an elderly couple of gods, demanding them to sacrifice on of their daughters to him every year. Susanoo-no-Mikoto met the couple before they had to sacrifice their eighth and last daughter, Kushinada hime, where he offered to help them in exchange for their daughter's hand in marriage. They then built a large fence with vats of sake. When Yamata-no-Orochi arrived it drank all of the sake, becoming so drunk he passed out. Taking his chance Susanoo-no-Mikoto sliced the giant serpent into tiny pieces, so much so that the Hi River flowed with blood. By the time he was at the tail his sword broke, after inspecting it he found the sword Kusanagi no Tsurugi. He gifted it to his sister Amaterasu and was allowed back into heaven.
In some legends, Yamata-no-Orochi avoided being slain by Susanoo-no-Mikoto, fleeing from Izumo to Ōmi and fathered a child with a wealthy person's daughter. This child would later be known as Shuten-Dōji, the infamous demon lord of Mt. Ōe.