Adventure Game Studio (acronymed AGS) is a Windows-only freeware game creator, specifically for adventure games. It's a very popular way to create adventure games. The Chzo Mythos series is also created with it.
AGS is licensed on its own AGS license, which allows commercial games distribution.
Adventure Game Studio was created by British programmer Chris Jones. It was originally released in 1997 as an MS-DOS program entitled "Adventure Creator".
Jones was inspired by the apparent simplicity of Sierra On-Line's adventure game interface, specifically as showcased in Space Quest IV: Roger Wilco and the Time Rippers. The first version of Adventure Creator allowed users to create only low-res, keyboard controlled games.
As a result of the lack of completed games and engine features the user base was small, but the community grew slowly. Game developers started requesting more features so that they could create more complex games. Gradually as these requests were implemented, AGS became a more capable toolkit and it was finally possible to create high quality games with it.
After a long period of slow activity, Lassi Quest was released as the first complete AGS game in late 1999. It was not until the Larry Vales and Rob Blanc games had been released in 2000-2001, that the engine gained widespread popularity. There is now an active community containing thousands of members, and a large output of completed games of all sizes.
Ben Croshaw is one of the users of AGS. He has used it since he created the Rob Blanc trilogy, and the his other games after that, including 5 Days A Stranger and the following entries in the Chzo Mythos series, citing his familiarity with the AGS engine.
When he was creating Chzo Mythos, he constantly experimenting with engine's capabilites, especially on gameplay. This can be seen on the change of gameplay feature in Trilby's Notes, from the usual point and click (used in 5 Days and 7 Days) to interactive fiction-like gameplay, using only the keyboard to type command and arrows to move. However, in 6 Days A Sacrifice, the gameplay reverted back to the usual point and click in similar style found in 7 Days A Skeptic.