Oni is a type of yōkai, demon, orc, ogre, or troll in Japanese folklore. They are believed to live in caves or deep in the mountains and are known for their superhuman strength and evil nature, manifesting in their propensity for murder and cannibalism. Oni have been associated with powers like thunder and lightning. They are popular characters in Japanese art, literature, and theater and appear as stock villains in well-known fairy tales.

The name "oni" comes from the Japanese reading of the Chinese character for ghost, and it is thought that the term "oni" actually referred to any spirit, monster, or ghost, but it gradually evolved to mean the oni we know today. Traditionally, Japanese oni are famous for their extreme wickedness and are prone to seize every opportunity to cast spells and inflict suffering upon their unfortunate victims. They are known for their strength and will use that strength to take over shrines or villages.

Although oni have been described as frightening creatures, they have become tamer in modern culture as people tell less frightening stories about them. Recently, there have been some excellent stories about oni, depicting them as creatures who only want a bit of company.