The Upside Down

The Upside Down is an alternate dimension existing in parallel to the human world. The dimension originally consisted entirely of sprawling mountains and floating rocks, and was home to a race of humanoid predators. Somehow, the Upside Down transformed from its original state and became a perfect copy of the human world, exactly as it existed on November 6, 1983.However, unlike the human world, this new incarnation of the Upside Down was overrun with alien vines, spores and membranes, and completely devoid of human life.
The alien vines, along with the humanoid predators and a species of bat-like creatures, formed a shared hive mind. This shared psychic connection was made possible via "the Mind Flayer", a powerful entity native to the Upside Down which Henry Creel possessed control over. The entirety of the Upside Down was essentially one gigantic organism, with all its contents and agents connected both physically and mentally via Henry and the Mind Flayer.