Another local legend (and the guy who told me this story is not one to tell tall tales...) - Donskoj monastyr was allegedly used by the NKVD during the purges to execute people (as a prison so to speak). People who lived in the area described seeing stray dogs trotting out of the complex with human body parts in the mouths.
I know this is where Patriarch Tikon was held when he was arrested.
His relics are there now. I was able to venerate them during my visit. (he was very small!)
It wouldn't surprise me that the NKVD would use a beautiful and peaceful place like Donskoi for such horrors.
I remember telling my brother during one of our walks through Moscow that I could feel the presence of all of the martyrs around every corner.