I do not own this mod. I am simply putting it up here so that other people may have easier access to it.
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Just to reiterate, I do not own this mod.
Mystery was originally a mod created to have the same atmosphere as the mod "MISERY" created for Call of Pripyat. It didn't change anything gameplay related however, just textures, UI, particle effects, etc.
They stopped working on it, but several Russian modders added on to it and released it back in 2014 with apparently significant gameplay changes. I was able to get it to run on a normal English GOG copy but I was getting pretty bad lag and some strange UI bugs. Just drag and drop the gamedata folder into your directory like usual if you want to try it out anyway.
On the Stalkermod.ru website translated to English, they detail:
As well as many other changes. If someone wants to take a crack at fixing any apparent bugs, feel free to do so.