100 years in the past
A century before the beginning of the game, Wakka and the Celestials fled there tribe on the Moon to the Celestial Plane, where the gods lived, in the Ark of Yamato. However, True Orochi and a hoard of demons ambushed them. At the same time, Yami and other evil creatures that were hiding inside the ark starting massacring the Celestials. Only Wakka managed to escape by crashing the Ark of Yamato on Earth, but then Orochi, Yami, the demons and all other forms of evil spread around the Earth.
Orochi took refuge in the Moon Cave and started forcing the residents of Kamiki Village to sacrifice young women to him as food or else he would destroy there village. After many sacrifices, Amaterasu, the reincarnation of the sun god Shiranui from a century later appeared in Kamiki to save a villager from Wep'keer who wandered into the Spirit Gate and into Orochi's lair in the past. After Nami was marked for sacrifice, Amaterasu dressed the great warrior, Nagi, as Nami and took him to Orochi's lair, in which Amaterasu and Orochi began a long battle, which Ammy won using the 8 Sacred Sake to immobalize Orochi. Nagi then used a powerful attack, which was made successful with the help of Amaterasu. It seems as if Orochi has been slain, but it survived Nagi's vicious attack and tries to kill Nagi by crushing him under a boulder, but the real Shiranui comes in and saves Nagi. Because of this, Orochi is sealed for a century and Shiranui is killed.
During the present of the game
A century after his defeat, Orochi is released by a descendant of Nagi, Susano, who wants to prove that Nagi's defeat of Orochi is a myth, because he hates how people expect him to be a great warrior. After this, Orochi unleashes a huge evil power on the world, cursing the lands, killing most signs of nature, allowing demons to roam free and paralyzing people. However, the sun god Shiranui is reborn as Amaterasu and begins restoring the land. During Amaterasu's adventure, Orochi haunts Susano's dreams, trying to convince him to pledge his loyalty to him. Orochi itself doesn't do much throughout the entire game, until the Kamiki Festival. There, Susano drinks all the sacred sake, infuriating the villagers. Right at that moment, Susano tells them he released Orochi, and just then, the monster marks the house of the sake brewer, Kushi. She realizes she must stop Orochi or the entire village will be destroyed, and gets more sake and then has Amaterasu ride her to the Moon Cave, where Orochi prepares to consume her, but is then challenged by the sun god once again.
After a long battle, each of Orochi's heads are injured, and Susano appears on the scene in full armor. Orochi tries to convince him to pledge his faith in him, so they can both obtain godly power over darkness, but Susano denies his offer, and with the power of Amaterasu's Celestial Brush, slays Orochi, with a spiral of darkness rising from his corpse and flying into the distance.
Later in the game, after Amaterasu travels back in time to kill Orochi with Nagi, the sun god fights Orochi again in the Ark of Yamato before confronting the god of darkness, Yami.
Orochi is a eight headed demonic serpent of massive size. On Orochi's back is a shrine surrounded by a garden with a giant temple bell hanging in it on a large stone pedestal. Underneath the pedestal is a mass of writhing tails, entwined, and at the sides of the mass, sprout eight necks the size of tree trunks, and on the end of each neck is a head, wearing a samurai helmet with thick armor plating and armed with a gaping mouth covered in vicious teeth, and on the outside is lined with barbs pointing outward. Each head of Orochi has an element, clockwise: fire, earth, poison, light, lightning, water, wind darkness. The samurai helmet of each head bears the emblem of its element and the element's kanji: fire has the emblem of a raging flame; earth has a large mountain emblem on its helmet; poison has a purple skull and crossbones; light has a circle, with shining rays projecting from it; lightning has a lightning bolt, with the kanji on the middle of the bolt; water has the emblem of a water drop; wind's has the symbol of a whirlwind, which is a circle and two spirals sprouting from it; and darkness has its emblem of a crescent moon in complete black.