Liminal Space Survival: Anime Series.

The backrooms or the liminal rooms, such an enigmatic never-ending place. Most of the lost souls there are normal people with average capabilities and most of the time, they die. They don't last long and if the backrooms won't consume them, their own psyche will. No one can truly escape the backrooms, there is horror in infinity. However, what if the lost souls this time aren't your usual lost souls? What if they're familiar, what if they're characters from a series that you're fond of. Will they survive the liminal space? Their stories will connect and they'll handle the liminal rooms in their own way. Most of them will reject the backrooms, unwilling to surrender to it. Some will conquer it and even outright own it. — Description from novelcodex.org