Ao Oni

 is a freeware role-playing horror video game. The game features puzzle and RPG elements, and revolves around a teenager named Hiroshi, who is trapped with his friends in a haunted mansion, and is stalked by a blue demon.
Plot.
The player assumes the role of 'Hiroshi', a young teenager who enters a haunted mansion with his friends, Takuro, Takeshi, and Mika (along with Kazuya and Ryota in the original). Shortly after entering the mansion, the doors lock behind them. Hiroshi doubts that the mansion is haunted, but soon finds himself being pursued by an enigmatic blue Oni. Hiroshi must now find a way to save as many of his friends as possible and escape the mansion alive.
The Oni.
Very little information is explained in the game about the Oni and his reasons for inhabiting the mansion, but it is assumed that he may have been one of the mansion's old residents. The Oni is a muscular, tall humanoid with a large head and blue skin who is roughly twice the size of an average human. When he appears in the game, a characteristic "shrieking" string chord plays until the player outruns him.
Normally, his mouth is kept closed in an amused smirk, but in a secret ending he is shown to have a set of sharp, knife-like teeth. If the Oni captures and kills one of Hiroshi's friends, they too turn into an Oni, distinguishable by their hairstyles, which they kept when transformed by the Oni.
In addition to the standard Oni, the player can enter an "Oni Room", containing numerous deformed, mutated, and completely original Onis, including a rectangular oni called "Blockman", who resembles the popular Japanese character Domo-kun, and a gigantic, muscle-bound Oni who humorously appears to be hefting invisible weights when encountered for the first time. Both of these Onis can chase the player more than once if the player enters their names when creating a new game.
In general, the Oni will chase the player at random after he has first been seen, but several of the chases are necessary to advance the storyline.
Gameplay.
During the game, the player explores various rooms of the mansion, gathering items and solving puzzles to help them progress. At seemingly random points, the Oni will appear in the room, with the shrieking string chord accompanying them, forcing the player to evade the Oni, either by outrunning or hiding from them. Most of these chases can be avoided by carrying out tasks correctly or in the right order, but there are several chases which must be carried out to advance the game's story.
Media.
Novels.
Novelizations.
In 2013 - 2017, a series of novels were released which were based on the game. Slight differences to the storyline, mostly based on characterisation are present, but the books are otherwise faithful to the game's plot. The title was licensed for digital distribution in the English language by J-Novel Club. The series is made up of five novels, which J-Novel Club released between 2018 and 2019: "Ao Oni", "Ao Oni: Vengeance", "Ao Oni: Mutation", "Ao Oni: Grudge", and "Ao Oni: Forever".
Junior novels.
In 2018 PHP Laboratory began publishing a series of junior novels based on the "Ao Oni" series. The series follows the game characters, who are of elementary school age instead of teenagers, and is told from the perspective of a stray dog that was taken in by Takuro's uncle.
Live-action film.
"Ao Oni" was adapted into a live action film of the same name, that was released in Japan on 5 July 2014.
Anime.
An anime television short series adaptation premiered on 3 October 2016. The series is produced by Studio Deen, and directed by Toshirō Hamamura and Chisei Maeda, with Kanekoke handling series composition, Kichi designing the characters and Shiyu Yanagida composing the music. An anime film, also produced by Studio Deen, released in 2017.
Reception.
The independently developed freeware game gained a cult status online in Japan, gaining notoriety on websites such as Niconico and YouTube where "Ao Oni"-related videos exceeded 50 million views in 2013. The popularity of the game revolves around the effectiveness of its horror aspects despite being a simple game, with the background music often being cited as a key contributor to the chilling atmosphere used to incite fear.