You are in the underworld. It is an afterlife, and it is dark, but you are unable to tell if it is a punishment or it was an inevitable one-size-fits-all afterlife for humanity. You lack specific memories of the world you come from, but you have some general knowledge.
As you reside within the underworld, you become less human, being that part of your subjection to the underworld is to lose yourself, at least to some degree. Depending on the mix of places you are, for how long, and what you do there, during the time in which your metamorphosis takes place, you will gain a new nature. However, once you have fully acquired this new nature, you will be resilient against further changes.
The first place you will find yourself is the junkgarden. It is a grey place, full of dust and incomprehensible ruined objects, as indistinguishable as the objects in one of those stroke-simulating pictures. It is perpetually in a state of murky morning, the pale brass sunlight above being half drowned through the silt that seems to hang in the atmosphere.
Through sudden cracks in the ground, or through more horizontall but generally downward-sloping departure, other segments of the underworld may be entered. These seem like other worlds, and are darker, but freer to explore than the constraint of the junkgarden. Oftentimes the sky, if still visible, will be starless, and if not black will be a deep red or purple, or an unnatural kind of blue. Most ground is either stone or silt, and where there are people, or what passes for them, there is usually confusion – people either sending other or declaring errands that the change their story on the nature of, group-based distractions for the unchanged to hook them into the collectives of certain places by making them change, lore dumps of varying levels of accuracy, etcetera.
The underworld is not hell, strictly, but it cannot be called purgatory because there is no intended way out. Once down there, whatever cosmic order sent you there intends, if anything, that you only descend deeper so as to be totally disposed of, not even taking up space at the top so that more refuse can be dropped down.
The underworld seems either to connect to multiple living worlds, or to contain reflections of previous ages in which the world was vastly different, as beneath the top layers inhabited by being with familiarly modern personalities, there are quiet, serene worldlets that seem to palely reflect a simpler, but more fantastical world. In the case of the latter hypothesis, maybe the magic of that age has itself passed, in some way, to these quiet refuges, which are haunted by liches and classical-looking demons.
There are some coherent corporate agencies in the underworld, with cohesive philosophies, or something resembling them, principles, or at least the image of them, and modes of operation. The deciding factor that seems to distinguish these and the normal rabble is leadership; more specifically leadership by true demons. These are creatures in the underworld, serving as its backbone, that were never human. They serve only to aid in the ‘corruption’ and habituation of human (and, subsequently, ex-human) denizens of the underworld.
It is possible for one to resist the change, through sheer willpower, as well as to acquire power without changing, in the underworld. These two things are connected, but not synonymous. Holding fast initially requires and grants no special powers, and it is also possible, through sheer exertion or strength or esoteric methods, to gain power unrelated to the struggle, but in some cases a powerful mastery over a potential demonic self through the will can produce powers something like that of that potential self within the one resisting.
It is highly likely that there is a devil underneath it all, ruling over all in the underworld, but he is subject to the forces that sent you here. It is possible that reaching the bottom and destroying him will, if not destroy, then dramatically fracture and/or destablize the underworld, or at least sap it of its central, oppressive vitality.