The Chzo Mythos is the collective title given to a series of four amateur adventure game created by Ben "Yahtzee" Croshaw using the Adventure Game Studio development tool. The games are, in order of release and intended play-order, 5 Days a Stranger, 7 Days a Skeptic, Trilby's Notes and 6 Days a Sacrifice. (In chronological order of the games' events: 5 Days, Trilby's Notes, 6 Days, 7 Days).
The series is also sometimes called the DeFoe series, after one of the main characters; the Trilby series, after another main character; or the "X Days a Sauerkraut," based on Croshaw's own references. The author commentary for 6 Days a Sacrifice confirms Chzo Mythos as Yahtzee's intended title, despite referring to it several times on the official sites as the "John Defoe Quadrilogy".
In 5 Days a Stranger, the player controls the shady cat burglar Trilby, who stumbles across a demonic force that manifests itself as a masked killer in the tradition of Jason Voorhees or Michael Myers, while finding himself one of a group of strangers thrown together in an abandoned mansion, inspired by Nocturnal Illusion, and being picked off one by one. 7 Days a Skeptic emulates the claustrophobic horror of Alien, following a spaceship crew that finds artifacts from the first game floating in space, four hundred years after the events of 5 Days a Stranger. Trilby's Notes, set in a hotel which exists in both the real world and a horrific alternate dimension in the style of Silent Hill, goes back to flesh out the origin of the cursed African idol from the other games. While the first two games use the point and click interface typical of recent adventure games, Trilby's Notes requires the player to move with the keyboard and type commands with a text parser, similarly to early Sierra On-Line games like King's Quest I-IV. 6 Days a Sacrifice completes the set, sitting in the timeline exactly halfway between 5 Days and 7 Days. Yahtzee's later game, Trilby: The Art of Theft, features the character's exploits before the series, but is not connected to it thematically, story-wise or gameplay-wise.
The games have featured on various PC magazine cover disks, and were mentioned as an "excellent series" and given a brief review in an article on Adventure Game Studio in the February 2006 edition of PC Gamer. 5 Days a Stranger is mentioned as a good example of a game created with Adventure Game Studio in the book Gaming Hacks published by O'Reilly Media
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Official Site: Ben Yahtzee Croshaw
Links: Chzo Mythos Wiki Five Days a Stranger Trilby's Notes