What We Know We Know
There is a Bad Place. Or more accurately there are bad places that do not play by the rules of our universe. Doors and portals and gates and tunnels open up to these places that are dangerous and alien. Most of these doors pop up like mushrooms in the neglected corpses of buildings where once people lived and worked and writhed together in the squalid theater of life.
These domains are connected for the most part. The knowledgeable, determined and lucky can find and navigate the doors between them. The key that seems to tie these domains together is that they seem designed to capitalize or at least endorse the suffering of their human residents. There is no real comfort or serenity within these places, even to the creatures native to its cruel physics.
There are new rules in each one. The Laws of Nature are often absent or mutilated to a degree they become unrecognizable to their ablest observers. There are laws to be sure; repeatability and ardent study occur often, but mechanisms for many are never found. We know the lost can "survive" decapitation in the Factory and sprinting is markedly slower than walking in Dust for everyone, but we do not know how. Some Underlaws effect only the dead people that are in residence. Some effect everyone that enters.
Dead people show up there. People that die in our world go to these places. At least, the bad people do. If the point of the domains isn't punishment, then they do an exceptional job on the side. Torture isn't just the language of natives, it is woven into the blood and marrow of the location itself. It is a unifying physic. Some domains seem to contain only lost souls that have a similar history or catalog of actions; others have vastly diverse backgrounds among it's residents.
These lost souls can be brought back. By bringing them back through a door to Topside, those who were once dead can live again. Their old body is unaffected by this process and it remains as it was. Their new body undergoes no change, save for the reassertion of our Laws of Nature. A severed head may even be able to talk when you fish it out of the Offal Pit, but once it crosses the threshold to Topide, it will die as quickly as any other severed head.
You can die there. Those traveling through these places can and do die often. They have not yet reappeared in the same place or, in fact, anywhere. There are several hypotheses that there are special bad places just for interlopers or "crashers".
Shackles. The lost are restrained to these locations in ways living trespassers are not. Often the restraint or shackle is provided by a sentient or semi-sentient group of entities, such as the crows of the Lonely Woods. Other places, the shackles are an intangible aspect of the environment. Regardless, these shackles cannot travel from domain to domain. The crows explode if the cross a threshold between domains; Thoughts of the compulsive need to search evaporate after one leaves the Silent City. Shackles are rarely a threat to crashers, but their only purpose seems to be keeping the lost in residence and at the mercy to the domain's tortures.
Screws. Beings known as screws are less predictable than shackles. Some are fanatically devoted to the suffering of the lost while others are unconcerned with anything beyond their own pleasure. Screws are very often dangerous but can often be overcome with skill and firepower. They are sentient and tied to their home domain. Despite this, they can travel to other domains, but crumple to ash instantly upon coming into our world.
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Demons. Demons are much like screws, but with an inexplicable interest in particular events topside and they are much, much more powerful. Demons maintain a respect and implicit authority among the screws of their domain and are considered aristocracy of a sort to most crashers. Demons can be bargained with, but they are very skilled in negotiation and have powers that can often manifest Topside. They cannot travel topside themselves and so have been known to invest lost or even crashers with a modicum of their power to act in their interests. These people are known as hachetmen.
Hachetmen. Hatchetmen are the bogeymen of crasher culture. They are sent to reclaim escaped souls, usually by rekilling them. Sometimes killings assumed to be the result of a hatchetman include esoteric and bizarre ritual elements. Sometimes hatchetmen are sent after crashers themselves if they draw the attention of a hatchetman's patron. Hatchetmen often have powers or strange abilities, but are more dangerous for their desperate dedication. Their contracts with demons rarely leave room for failure.
What We Think We Know
Everyone there deserves to be there. No one wakes up Down Below for being an atheist, or for being gay, for shooting the guy who raped their wife, for getting an abortion, or for getting drunk on the communion wine when they were 16. Everyone there committed at least one act of genuine, true evil in their lives, be it taking advantage of a drunk girl at a college party, being a serial poisoner of neighborhood cats, or “Just following orders” at a Serb rape camp.
No one has met an angel. If there is an opposite to these bad places, no one has come back with anything resembling evidence. Every crasher can give detailed accounts or even show you pictures of demons and damned, but not once has a reliable account of an angel popped up. Priests, rituals and magics from every culture have repeatedly failed to discomfort the natives of these domains. While most religious traditions can shoehorn the findings of crashers into their own dogma, no one's dogma has really come out on top of the list for metaphysical accuracy.
What We Don't Know
Who’s in Charge? Somehow, between death and appearance in a domain, the lost are sorted into groups. People who willingly infected others go over here and war profiteers go over there. By what mechanism or judgment this occurs has never been determined. The lost have no memory of an intermediary stage between death and damnation.
If justice is a goal of these places, they seem to operate by an inefficient and largely apathetic binary rubric. Once the lost appear in a domain, they are almost always anonymous. Whatever crimes landed the dead in that location don't seem to concern the screws in any way.
The doors between the domains that crashers use are not the only way to move between the domains. There are train systems and bus lines and highways that travel non-euclidean routes from one domain to another. These Transits do not exist for the convenience of crashers. Who established or maintains these Transits is a complete mystery. Demons will give any answer to these questions they think will benefit them the most and cannot, as a hard rule, be trusted. Many believe the evidence suggests that demons simply do not know.
What are the doors for? No shackles, nor screws, nor even demons can use the doors. Crashers are not welcome and the lost are successfully prevented from using the doors. So why built or place a fixed portal? There seem to be metaphysical locks of a sort to prevent the casual use of some, but these aren't universal or even difficult to overcome. As with everything in crasher society, there is much debate and hypotheses, but none yet that cover the full facts.