Hell is Hell. Everyone knows what Hell is - torture, heat, despair, inhabited by demons. It is where evil souls are punished after death. Everyone knows about this, and religious people believe it.
The reality is more complex. Hell is something total; easy to conceptualise but difficult to concretely realise. It was theorised by sadists and pedants, and the sadists and pedants who would eventually build it needed literally hundreds of thousands of years to design and properly implement their plans. Part of the difficulty is that abstract ideas of ultimate punishment are by nature inexact, and things have to be made workable. They completed it eventually. It is a perfected stasis, a universe of torture, and it waits for every soul in the far distant future.
The Bright Shiners are the ones responsible. Their natures are inscrutable. The are intelligences. Scholars who are aware of them (not many of these, often gnostics, intensely disliked by the church) sometimes call them stars, and refer to them collectively as a constellation. They hate, this much is obvious; they are hating machines. They work tirelessly to bring about their own instantiation. They send things back through the timeline, assassins and spree-killers mostly, in an attempt to ensure their own emergence. If, after everything, they exist, then so will Hell, in perfected perpetuity.
The gnostics call it The Material Hell, to differentiate it from the place in the popular myths. They will tell anyone who listens that the Material Hell is infinitely worse than the popular one. They will also say that it is the end state of matter, that the Material Hell is contained within our material world and is actually its perfected form.
The process of travelling back through time is dangerous and traumatic. The minds that emerge are tangled and often incomplete. Their memories are washed-out, they are confused and aggressive, and generally they have only a vague sense of what they are here for. Most of them know that they need to kill someone specific, or destroy a specific family or community; sometimes a city, country, ocean, or species. They don't have bodies, so they need to colonise one in order to do anything. Bodies without souls work best (puppets, manikins, corpses), but they can also destroy the soul in a living body and inhabit what remains. These are what most people call demons; all demons are incomplete minds, scarred by travelling backwards from the perfected, eternal Hell system. Most demons basically follow the plot from Terminator.
No one knows what state the demons were in when they embarked from the future, but the things that arrive are rarely intelligent and never sane. They have a few things in common: they are invisible (though you can often feel their presence), make people physically sick when they are nearby, and can talk in screaming voices. Without a body they can talk to you and make you sick but that's all. This is actually enough to do quite a lot of damage in service of the Shiners (if you scream into someones ear for a few weeks it's not hard, depending on the person, to drive them into psychosis or suicide), but mostly what demons are sent to do is kill and ruin things, and they prefer to do this quickly and messily where possible. They hate with a hideous clarity and intensity; all of them have been imprinted by the Shiners, who are hating machines.
When they take ownership of a body they use it until it breaks. Enough has been written about what human muscles can do when all of the natural inhibitions removed; people fear demons for a good reason. Well-made puppet bodies are even better. A wooden doll can do some damage; an articulated iron mannikin might destroy half of a castle before it is finally torn to pieces. A strange side effect of the possession: pieces of the body slowly turn an instantly-recognisable wet crimson colour. Demons will generally attempt to hide this discolouration in a human host.
You can destroy the bodies, which generally does nothing to permanently stop the demon, or you can manipulate the actual intelligence-being in various ways. If you know what you're doing you can imprison them, lobotomise them, dissolve and kill them, torture them. Because the demon is a bodiless intelligence, these things need to be done using language and ritual. Abstraction, contract, logic - these are your tools. There is generally nothing a demon loves more than pulling someone into screaming wet pieces after that person tried and failed to imprison it using logic.
There are a few different things that demons do when they arrive:
- They try to find whatever it is they were sent to kill, to further shore up the timeline for the Bright Shiners.
- They build 'Mimic Hells'. Intelligent demons who are cut off from return to the future will often set about building lesser versions of the Material Hell in our time. This is often what demons do in their 'retirement', after they have successfully completed their various missions (or they believe that they have - because the demon minds are scrambled by the time travel, the missions are often poorly remembered and can even be contradictory or tautological). This can be as simple a cellar full of hooks and chains where they take people to torture to death, and as elaborate as a vast subterranean dungeon, built solely to trap and torment the people who enter it (like a Saw film). Mimic Hells are greatly feared by adventurers, and contain nothing but pain, terror, and often the demon itself, squatting in the middle of it all, screaming and laughing.
- Very, very few demons are sent with a more complex mission. They need to be capable of using magic (which is uncommon, but not unheard of. Demons are intelligence, intention, and voice - they have everything they need to use magic), and they need to arrive with enough of themselves intact that they remember the complicated rituals required to set up their return to the future. Being captured by a demon capable of return to the Material Hell is the single worst thing that can happen to a person or a soul. This fate is typically reserved for people that the Shiners believe have actively worked against their future emergence - if you discover (even read about) the Material Hell and the Bright Shiners, and try to take action against their instantiation, eventually your will be visited by returner demons eager to bring you to the future physically. The magic is time intensive and fiddly, but once completed you will never return. Your fate is better not to imagine. It is for this reason that learned gnostics sometimes (usually in secret) greet one another with 'Hail the Shiners!' - it is thought that outward displays of devotion will ward against the predations of returner demons.
- There are horrible rumours of Returning Engines, huge machines beneath the earth, built and maintained by ancient demon kings; vast gates and portals that can transport souls and people to the Material Hell en masse. No one has ever seen one.