Some of you may have seen the new Netflix series "Stranger Things", where tabletop gaming specifically D&D made a very cool apparition.

So, don't you think that it would be great if Demogorgon was present in D:OS2?

Demogorgon is not D&D exclusive so it's not copyrighted. It has appeared in many works of fiction throughout history:

https://en.wikipedia.org/wiki/Demogorgon

Fire and wind come from the sky, from the gods of the sky.
But Crom is your god, Crom and he lives in the earth.