|Demon, riding Devil.|
Demons are the embodiment of everything that makes us fuck each other over. They don't embody things wholly without merit but there is no such thing as a temperate Demon. Some Demons are referred to as Devils, but of course not all Demons are Devils. There is a difference, and it's not where their HOUSE is, it's not in whether they serve Goat Pig Satan or Wizard Satan, it's not in which edition of the Junior Woodchuck Guide Book they use.
Demons embody sensations. There are fire demons, bubbling acid demons, ripping hook demons, demons who fuck you, demons whose foul brimstone stench is palpable and caustic, demons whose speech makes your ears bleed, demons of tenderness who are the worst of all, demons of rotting while alive, demons who eat you, and so on.
Devils embody concepts. Wrath, Lust, Idolatry, Depression, Infanticide, Madness, Persecution, Lies, Profit, Hope Devils who are the worst of all, Hunger, Being Lost, and Power.
In The Hell (for all hells are really one hell, and the mind can make a hell of heaven, and every hell is really a thousand hells, yadda yadda) there is the horrible chaos and there are pyramids of fiendish glory and the twain meet ALL THE TIME, each group fair crawling over the other. The only way you'd know the distinction as a damned soul is that your experiences would change. Otherwise they're all just horrors.
A Demon will possess you and drive you like a car and you will do horrible things and suffer for them for all eternity as those same things are done to you.
With a Devil you'll still do all those things but the Devil will make it seem like your idea. You will be coerced into trapping yourself, walking into said trap, and slamming the lock on your own trap firmly in place.
A Demon can possess anyone at any time, and fuck bargaining.
A Devil can possess anyone at any time but won't. Devils prefer for you to set the terms by which they come to you, then fuck you on your own terms, THEN set the terms by which you try to barter back your life dreams and soul.
Demons come into animals and make them into horrible things and set them loose upon the wood.
Devils would never possess an animal, and typically controls them or enraptures their attention to do its bidding.
Demons always look horrible but are not obviously from Hell at a distance.
Devils may look beautiful but it would be a kind of dark primeval beauty more chilling than gore, whose provenance is never in doubt.
Demons hate everything, but they chiefly fuck with Clerics and divine spellcasters because the rules of their deities limit their sensations. There's always SOMETHING. And if nothing else they rob death of its right due, which is a crime against dying, and therefore sensation.
Devils want to fuck with everyone like Heath Ledger but they particularly like fucking with Wizards and other spellcasters because any knowledge of the unknown or forbidden must be sought and is like you're begging to be fucked with. They ostracize certain concepts as well, such as the concept of divine spellcasting. Really though spellcasters are just EASIER, and any Devil would rather be about its business sooner than later.
Demons are simple.
Devils rest in details.
Demons like the races for supper.
Devils like the races like I like RPGs.
Demons are the Act.
Devils are the Will.
There is another Helldweller not either of these but both, the Asura. Asuras are eternal spirits whose own extremes in both Thought and Deed have turned their eternity dark and horrible. They would die as many of them have died and go on to some reward or cease to exist. These Persist, and in Persisting they have no true place in the waking world or the glories of heavens. These are fallen angels. These are Stalin and Dracula (when everybody calls Dracula a demon this is what they mean). They aren't all bad, and that's what's worse. They are beings who could have become or stayed angels, who might have been saints or gods, who are now something less, something more, something OUTSIDE. They will possess their old forms but usually don't, preferring to be physically manifested as themselves by old acolytes and young believers.
And of course there are the Archdemons, the princes and queens of black ethics and red sin. Pig Goat Satan and Wizard Satan and Spider Lady and Patches and Doublemonkey are all here. They're too big to possess you, spiritually speaking, the same way a river's too big to fit in John Lithgow. Their best bet is to make you near-god already and then take your form, become like a thousand normal mortals, or save up their power enough to physically manifest when the forces that oppose this (gods, nature, PHYSICS) are weakest.
In Hell there is a tower. Within the tower are creatures who have telescope teeth, who constantly stoke penis furnaces. They are watched for fun by grinning letches who promise you the pain is over, maybe you'd like to try THIS door, maybe if you do THIS we'll let you go. Through all this chaos, untouched, genocidal kings and plaintive clicking steel rub shoulders with storied necromancers, drinking from their #1 Rapist mugs and plying their crafts as they did in life, empowered by Hell and free from mortal restraint. On top of the tower Pig Goat screams at a pile of skeletons so hard that some get up to come kill farmers and treasure hunters instead. To the soul in hell it's simply neverending suck. To the learned world it's like the difference between Hulks, in that there are distinctions but none so much that they really matter so it's all academic.
In an adventure nobody should Pokemon an encounter with the dark mirror of creation. They should go "What the fuck just happened?" and only from piecing together their bizarre experiences and narrow escape can they figure out, indeed, what did just happen. Like it's done in movies and stories.