6 Days a Sacrifice is the fourth and final game in order of release, and third in order of the game series' timeline, placed 196 years after 5 Days and Notes and 196 years before 7 Days. You play Theo DaCabe, a council surveyor who is trying to run a health and safety assessment on the headquarters of a fictional fad religion called Optimology and gets embroiled in the machinations of the overarching series plot.
The Special Edition includes author commentary, an "expanded, slightly less ambiguous ending", the soundtrack in midi format, and character profiles on most of the characters in the series, in the guise of dossiers from the Ministry of Occultism.
The game also has a series of accompanying interactive fiction games, called the Countdown Trilogy, which were used as teaser trailers for the release of the point-and-click graphical adventure itself. They used the Z-machine language for implementation and need an interpreter such as WinFrotz to run. These games were developed using the Inform7 development system. Each of the three text adventures is named after and represents one of the three Blessed Agonies, an important plot element in the second half of the main series, and the three aspects of being, important throughout the series.
6 Days a Sacrifice won Best Story and Best Non-player Character (Trilby) in the 2007 AGS Awards, having received a total of 7 nominations. This brings the total number of AGS Awards for the series to 13 and nominations to 36, although The Art of Theft, not strictly part of the series, received a further 2 awards.
Source: Wikipedia, "Chzo Mythos," available under the CC-BY-SA License.