Abyss
Limbo (also known as the Sewers or Compost Heap) is the term used for a layer that exists to catch things that fall through the cracks in the world. Others lurk inside limbo, as well as people who have had their connections to the world completely severed or when whatever harvests the soul of the dead refuses to take them and a sluice from above drops them in.
The sky above is nothing but sheer darkness that has a weight and presses down, making you want to crawl in the mud beneath your feet. Bits and pieces of things from the real world could be found there too. It's a constantly evolving maze that's only growing as more and more things fall through the cracks, but the different parts are connected to one another via tunnels and small lights.
Inside the realm, you are what you eat and lose your sense of Self. It's entire purpose is to break whatever falls there and, if you want out, the only way is to go through by changing, adapting, eating, and getting stronger.
There are places where it’s closer to the real world than others. One part, you have to swim against a current for a while without a chance to surface. Other parts are guarded by really strong things that eat you if you're too weak or make you agree to carry something back.
The only way to get back while maintaining your sense of Self is for someone to enter the realm and carry them back, but they usually only seek out the worst monsters since finding someone is specific is difficult.
Blake Thorburn hypothesized that Bogeymen were born here.