Chaos;Head

Chaos;Head is a visual novel game in which the player takes the role of Takumi Nishijou, an otaku and shut-in, who experiences delusions. The game is mostly linear, but frequently includes player choices: the player is able to choose whether the hallucinations Takumi experiences should be positive or negative, or if he should manage to stay grounded in reality. Positive delusions generally involve comical or erotic scenes, while negative ones include horror elements and violence. The player chooses the delusions through the "delusional trigger" system, wherein green and red lights are displayed on the top of the screen, representing positive and negative delusions.

After the player has played through the game once, delusions chosen during subsequent playthroughs determine which of three endings they will reach. When the player replays the game, they are able to fast-forward past passages of text they have already read. In the updated re-release Chaos;Head Noah, further endings are possible to reach, also by choosing what delusions to experience. The player can read about key vocabulary used in the game, including internet slang, through the "TIPS" glossary, which is updated with new items as the player encounters them in the dialogue.

The game is set in 2009 in the Shibuya district of Tokyo, where Takumi lives in a cargo crate on top of an apartment building. He discusses the recent "New Generation Madness" ("New Gen") serial murder case in Shibuya with his friend Grim over the internet, when someone with the username "Shogun" sends Takumi image files depicting a man pinned to a wall with stakes. Later, Takumi witnesses a girl he does not recognize committing the murder portrayed in the image files, and he flees the murder scene. A few days later, she sits next to him in school. He thinks she will kill him, but is told that they supposedly have been friends for a year and that her name is Rimi Sakihata.

Convinced that Shogun is targeting him, Takumi tries to avoid getting involved in the murder case, which draws the attention of the police. As he becomes a suspect and more murders occur, Takumi worries that Shogun is targeting him and experiences paranoia and hallucinations, and becomes unsure of what is real and who he can trust. In one of his delusions, Shogun appears as an old man in a wheelchair and tells him that more people will die unless he awakens.

Chaos;Head was developed in a collaboration between 5pb. and Nitroplus, who describe it as a "delusional science novel" (妄想科学NVL, Mōsō Kagaku NVL). It was planned by Chiyomaru Shikura, the founder and executive director of 5pb., and was written by Naotaka Hayashi, with character designs by Mutsumi Sasaki and concept art by Yukihiro Matsuo. Shikura intended to make the story based on reality to make it more relatable, saying that he personally finds it difficult to get excited for fantasy stories.

The game's music was composed by Takeshi Abo, Toshimichi Isoe, Yoh Oyama, and Shintaro Jimbo, with Abo composing 21 of the songs. While it was common for games in the genre to have music that is played in several scenes, Abo had to compose several songs that specifically matched certain individual scenes that were intended to be shocking and dark. He described the main image for the game's sound as "rainy", compared to the later Science Adventure games Steins;Gate, Robotics;Notes and Chaos;Child, which he called "cloudy", "clear weather" and "stormy", respectively. In preparation for composing the music, Abo read the game's story to understand the setting and the character personalities as much as possible.

Chaos;Head Noah received "mixed or average" reviews according to review aggregator Metacritic.

The game was a commercial success, and, together with Steins;Gate, helped establish 5pb. as a game developer. One month before the game's original April 2008 release, Chaos;Head was the fourth most pre-ordered PC game in Japan. It debuted as the third highest-selling PC game of April 2008 in Japan; it became the sixteenth highest-selling visual novel game on the video game store Getchu.com during the first half of 2008, and thirty-fifth overall for the year. Chaos;Head Noah was the eleventh best selling video game of the week in Japan during its debut week, with 18,000 copies sold.