Orochi, or Yamata no orochi, (translated to "eight-forked-snake" in English) is a mythical beast from Japanese lore. It appears in two ancient texts, and in both versions he is slayed by Susanoo, god of sea and storm, after being expelled from heaven by Izanagi. Orochi is also always depicted as a being that requires sacrifices, and is slain before the eighth can reach him. According to the myths, it is killed by being made drunk and Susanoo recovers the sword Kusanagi from its tail, which is then presented to Amaterasu, his sister and Sun Goddess.
As one can ascertain by reading the history of Orochi, the myth ties heavily into mainly two video games: Okami and the King of Fighters franchise. The link to Okami is obvious, as the game was intended to be heavily based in Japanese mythology in the first place. Amaterasu and Susanoo both appear, and Orochi is slain in practically the same manner. The influence on King of Fighters, however, may not be so obvious to those who are not too big on the surprisingly complex lore of the franchise.
In what is known as the "Orochi Saga," taking place in KOF 95-97, Kyo Kusanagi, Iori Yagami, and Chizuru Kagura are the three fighters destined to take down the threat of Orochi. To make a very complicated story steeped in Japanese mythology short, Iori and Kyo's ancestors were in opposing clans in the past, while Chizuru's clan was neutral. They all ended up working together to take down Orochi, but Iori's clan made a pact with Orochi that strengthened them, but limited their lifespan. When Iori can no longer control his cursed blood, he becomes Orochi Iori, or "Riot Blood" Iori, a stronger and faster version of Iori with limited defense. In typical video game fashion, this is how they end up defeating Orochi (who was possessing the body of the New Face team's Chris' body), but nobody trusts Iori ever again.
For more on this version, see Orochi.