Not German

Source

During the 2008 financial crisis, my boss started buying up houses all over for like 40-50k each (he is a millionaire now) and it was my job to renovate them so he could rent them out.

Anyway, it was a bright sunshiney day and I was removing wood paneling in an upstairs bedroom and realized I had to get a Sawzall from our workshop (about a 15 min drive round trip) to cut out some old piping in the wall, when I came back I went to walk up the stairs and there was this black mist similar to what you described, I am a combat veteran and I can say I have been terrified before but this was something different. Not fear for my life but just like seeing absolute evil.

So I and a bunch of guys came back one night to do a voice recording and see if we could pick anything up. We picked up a man's voice that to us sounded like German (which made sense because this American city was founded by germans).

But a few years later I had a german girlfriend and played the recording and she said it did sound german, but she had no idea what he was saying. Pretty weird shit.