This started out as a combination of two ideas: changelings in the Cold War and a great Machine laying beneath Europe. Then I went and noticed a result from Seventh Sanctum's techno-fantasy monster generator, "motor changelings," and I was off.
Fairies are mechanical in some manner (probably more steampunk than cyberpunk, say, but who knows?). They live in the Otherworld, or Fairyland, but they come to this one because Hell is a power too, one which demands a tithe of them, "derived from the produce of the land." Specifically, every ten years they have to give up a tenth of the population increase. If we have one hundred changelings delivered into the world every seven years (a decent number, I think, especially if changelings/fairies live long lives) then one thousand fairies are "born" every seven years. Changelings are fairies who have been switched with humans so that they can be left to live on Earth instead of be delivered up to Hell.
Changelings, whatever their other attributes, always demonstrate superhuman strength and blur the lines between biological and mechanical. They are also unable to live in Fairyland for long; they have grown up in and become too well-adapted to the laws of the World.
The Machine Beneath Europe is a gigantic fairy. Not a changeling, an out-and-out fairy, one who remembers Fairyland. The characters, whoever they are, will travel through it at least once before they learn what it is.
Hell knows that the fairies are paying the Teind with humans. But they are totally fine with that, because changelings in the World make for superb agents. Hell can do some stuff in the World, sure, but they're limited in some way. So, unbeknownst to Fairyland, Hell is using the Teind as a roundabout way of getting agents into the World.
Maybe possession by a demon degrades a human's body. You can't play horse to a demon for long before dying outright.
Doing some stuff with philosophical zombies would be interesting here, making normal humans lack self-awareness, but then you have to wonder why Hell cares about a World of p-zombies.
Some stuff with superintelligent AI can be played with here, though. And if the fairies are machines, then what is Hell? What is Heaven?
Perhaps Hell is like the Flood. Or Tyranids. Or Zerg. They're raw flesh, basically. A disease, maybe even somewhat like a zombie plague. Maybe Hell isn't self-aware either. A bio-magical input-out system. It would be interesting if Hell were looking for souls in order to develop self-awareness-- self-aware demons are basically running on stolen software or something-- but that would be weird if normal humans were p-zombies. Maybe it's just one or the other.
If normal humans in the World are self-aware, like in the real world, then fairies like souls too. They trade souls like Pokemon cards. They trade self-aware minds, specifically. That's what a "soul" is: a consciousness. Demons like to run on human "hardware" but they aren't the dominant force in that case, more like an urge than anything else.
Fairies don't use souls as money. A twenty-dollar bill is worth twenty dollars no matter its condition, but one soul might be worth more to one fairy and less to another.
Heaven is like a cosmic force? Fairies are machines, demons are fleshy aberrations, and angels are living storms. Probably self-aware, like fairies and humans but unlike Hell.
Hell reads minds. They recognize people and things by mental signatures, and hunt these things down. You can't hide from a demon because it can smell your thinking. But fairies take human minds and then re-sculpt them, so to speak, so that they seem to be fairy minds. Hell is actually able to tell the difference, but the fairies don't know this. They don't know just how good the demons are at distinguishing different kinds of minds.
Maybe there's an "oh crap" moment when the characters find out, and realize that Hell must have been playing them this entire time.
If the Machine Beneath Europe is a fairy then apparently fairies can get really big. But why not the changelings? Maybe being in the World stunts their growth or something, or it's an intentional modification in order to make sure that they can always blend in.
Fairies run on some really weird programming. This is why they act esoteric and have such weird rules. Can they self-modify that programming? Maybe not.
Note: work fairy illusions into this somehow.
There is an element of untruth to these otherworldly powers (and "Otherworld" is hereby a term for Fairyland, Heaven, Hell, and any other possible place that is not the World). Fairies are not really machines in Fairyland. Demons are not actually flesh. Angels are not really disturbances of air and lightning and winds and fire. But this is how they manifest as they are "translated" into the rules of the World. If the characters ever go to one of these worlds then everything is going to get really crazy and nothing will be how they expected it.
(And maybe angels are also laws. The world starts operating differently when an angel is around, because the angel is, at least in part, literally a local change in physical laws).
Changelings make better hosts for demons. But they have to be willing? Or it is very hard to possess a changeling and requires lots of prep work? Or maybe it is possible for changelings to "tune in" on Hell's figurative radio frequency and communicate with demons without being damaged. Hell still has to negotiate with you, and mostly it just watches and waits and occasionally says something to you so subtly that, hopefully, you don't question where that thought came from.
It might be some time before the characters learn that there is a Hell and then realize that Hell has been talking to them this entire time.
Demons like to manifest out of people. Imagine them getting all chestburster and then growing rapidly, maybe even right before your eyes. Gore and screaming everywhere, and if you try to look at this thing that's come out of your friend then your eyes will hurt. Demons are a virus. They are contagion personified. They don't just get in your body. They will get into your minds. Demons can say things, horrible Lovecraftian ear worms that you can't get out of your mind, and those phrases that are uncurling and growing in your head are as much the demon Belphegor as the thing that said them to you.
Demons are philosophical zombies. Demons are philosophical zombies. They will eat your thoughts and they will eat your body and they will replicate. And sooner or later they will burn out because they haven't figured out how to survive in the World for long. It's like the World is a body with its own immune system and eventually it pushes the demon back out (and remember, the whole distributed system that started to spread from the incursion is one demon, a body without organs, and when the World is able to push out one part of the demon it is able to push out the whole thing).
That's why they need changelings. They can communicate with changelings. Sometimes they can use changelings as an incursion point, as a vector (more disease-y), through which they can spread. Demon figures out a way to translate itself into words and symbols, communicates this meme to the changeling and gets the changeling to spread it.
Memetic warfare. Demons have sigils in myths, don't they? Those sigils are the demons themselves. Well, the actual sigils that you know of are not the demons. When the World pushes out a demon, it pushes out the whole demon, including the memetic part. But somewhere along the line it was conveyed that demons often get themselves established in a symbolic form before they start infecting flesh, rock, buildings, and other things, and that's where we get the idea of demonic sigils.
Can you imagine trying to contain a demon? Abraxas is sitting there in confinement, all nice-like, talking with its interrogator, and... what's this? Abraxas is is tapping its fingers against the table, almost like Morse code but not quite. Those taps are getting into your head, slowly replicating. You go home, you say something to your kids. Except, you don't. You're just clicking your tongue. But you don't notice. You go on and check your email. You're changing, your body is twisting, but you don't notice that. You send out some emails to everyone at work, filled with some weirdass writing, but that doesn't strike you as strange at all. And that tapping that is repeating in your head is coalescing into words.
So, you can't confine demons. You can't deal with them. You kill them with fire, and then you find another five ways to deal with the remains, just to be sure.
Questions to answer:
- Why do the fairies have to switch out their young in the first place? Why not just grab human babies and be done with it?
- How did the war between Fairyland and Hell go? What was it like?
- Heaven needs to be addressed further.
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