The story takes place on Kisejima, a small island in the middle of the ocean. Here, it is said that the power that lies dormant within the local demon spirits can be drawn out by challenging them to trials, where ancient demons who were once defeated by Momotaro are summoned to battle. However, few have ever returned from Kisejima alive. Kuuta, the protagonist, is the only survivor of the Demon King's army that faced Momotaro in battle. He and Kazemaru, a mysterious spirit that lives on Kisejima, team up to take on challenges of all shapes and sizes! Kisejima may not be large, but the colorful artwork by KENEI DESIGN, with its dazzling green meadows, looming forests, and plains of floating megaliths, offers a unique emotional impact unlike any other action game. Its solo creator has invented a mini-world in which each and every location is marked by its own memorable appeal. ONI is an invitation for players to enjoy a small-world experience, for a change!