The parallel universe in Stranger Things is known as the Upside Down.
It's the Upside Down. It's first introduced in Season One when Will and Dustin are racing back to their houses after a Dungeons and Dragons campaign at Mike's/Wheeler's house then the Demogorgon captures Will and takes him into the Upside down. The Upside down was created by 001/Henry Creel/Vecna/ Peter, (technically was created by 011 when she killed Peter.) Who then created the 'monsters' this includes, the Demogorgon (S1), Demodog (S2 maybe, S3) , The Mind Flayer/ Shadow Monster (S3), Vecna and the Demobat's (S4).
