When reason sleeps monsters are born.Knock-Knock, a unique survival-horror adventure with puzzling mechanics, original cartoon graphics, and twisted atmosphere, is available 20% off for Windows and Mac OS X, on GOG.com. That's only $7.99 for the first week.The developers of Knock-Knock have chosen to shed some light on the unsettling origin of their creation, on their official website: In late November, 2011, a strange and troubling event has happened to our studio. We have received an anonymous e-mail which suggested that we should produce some "unconventional" game based upon the materials attached to the letter. Such offers are a dime a dozen and most of the time they barely deserve any attention at all, however, this one seemed quite different.The attachment contained a set of 19 files added to an archive titled "lestplay". The files (snippets of text, scraps of audio recording, video footage fragments) as well as the style and wording of the message itself appeared to be rather disturbing. The surface examination did not reveal anything straightforwardly terrifying, yet we could not escape the feeling that something truly sinister was lurking underneath.The stranger was begging us to complete the project he has started, yet he also made us well aware of the risk we took as the proposed ludic situation would potentially explain what calamity befell whoever has compiled the ominous archive, thus urging ourselves to meet the same fate.Knock-Knock is one of those games with a concept that may seem weird at first, but turn out to be really hard to put down once you start playing. In this bizarre survival horror title you'll be defending your cabin in the woods from the phantasmic "guests" that come out of the darkness and roam around the poorly illuminated rooms. Avoid encounters with the abominations of the dark, repair your home, and figure out how to survive a night. With twisted visuals and many interesting gameplay ideas will bring you both thrills and fun. Be careful though, it's dark in there and you might get eaten by a grue!Some doors are better left unopened, but it would be rude to ignore monsters going Knock-Knock, for only $7.99 on GOG.com. The 20% release discount offer lasts until Wednesday, October 16, at 9:59AM GMT.