What is the Vale of Shadows?


Now I’m sure plenty of us on here have seen Stranger Things recently (honestly I’m surprised I haven’t seen a question like this yet) and it’s whole premise about the Vale of Shadows has me intrigued. For those who haven’t seen it or don’t know, look here!

Based off of an earlier question of mine (here) I’d like to know what we currently know about the Vale of Shadows (or whatever it is actually called, as through some research, people have been calling it various different things). Are there any official sources that describe a plane similar to the Vale of Shadows and a little more specifically, what monsters live on that plane?

Ideally the focus would be on 5th Edition but I’ll accept any information available as I’m sure the premise of the plane can easily be converted.