The Reality Experiments or RE are pockets of reality created by the Demons of the old world. These makeshift worlds capture the souls of the deceased, molding themselves around their inhabitants' collective will, growing and evolving to fit their needs. Once a soul is trapped inside an RE, it begins a new life there with no memories of its past life.All Reality Experiments are different from one another and safe for a few special individuals, none of their respective populations know of the existence of the others. To a person living in an RE, it is their home planet/universe and nothing else exists past it.
The structure of its primary system resembles the structure of a planet; with a core, a somewhat hard shell which creates the landscape, and an outer aura of energy. This aura is called the shield and acts as a barrier which protects the RE from possible (but very unlikely) attacks from creatures of the abyss in which it resides.
It seemingly impossible for someone to go beyond the shield of a Reality Experiment (e.g., If someone tried to leave the planet by use of a spaceship, the RE would simply create more "space" in order for the traveller to be able to "explore"). However, there have been known cases of people disappearing without a trace; only to reappear many years later with no memories of what happened in between the moment they disappeared and the moment they came back. This hints towards a possible anomaly in the system; this argument is used by some as a way to prove the existence of other REs and the possibility of inter-RE travelling.