The Backrooms
The Backrooms were a “pocket reality between universes”. It had multiple “levels”, each infested with monsters.
Jenny Everywhere and Jenny Nowhere once ended up in the Backrooms after accidentally “clipping through reality” as a result of trying to chase each other through multiple rapid-fire shifts. In the first level, they were attacked by a shadow monster. Everywhere tried to shift the both of them out, but instead landed them on another level, resembling an endless corridor, where they were charged by a large group of monsters. Another shift brought them to a place that was simultaneously outdoors and indoors before Jenny finally managed to pull them out of the Backrooms and onto a busy street in a normal universe. (PROSE: Jenny Everywhere and the Nowhere Spiral)
Behind the scenes
“The Backrooms” are a creepypasta concept, which originated in a post in 2019. As its original creator posted it anonymously, the Backrooms are generally held to be in the public domain.