Demons and devils

I've got demons, devils and fiends, each representing a different nuanced version of darkness.

fiends are cunning, vindictive and cruel, these creatures recieve the lions share of souls darkened by sin and are the most relatable as humans, they fulfil many of the same actions and experience many of the same feelings, and it is this similarity which means that they are the most likely creatures to be able to learn the notions of sympathy and kindness, and fiends which grew too human in this manner lose their ageless nature and became the first tieflings.

devils are different, they play the long game. every action a devil takes has been carefully considered and weighed against all other options until the correct course is clear, devils have been fighting the gods for the fate of the world since it began, and they were the ones who introduced mortal evil into the world, this battle has become an all consuming obsession for the forces of darkness, and every action they take is devoted to the corruption of mortality for reasons long forgotten.

Demons are creatures of pure impulse, they come from a realm of pure emotion and thought where names and faces are discarded with regularity, nothing truly dies within the realm of demons, and even those who are devoured by greater beasts live on within them and escape whenever an opportunity is presented, so when they enter the mortal realms they behave in much the same way as they do within the abyss, heedless of the destruction they leave in their wake because they are as incapable of understanding mortals as mortals are of understanding them, legend has it that any demon who manages to comprehend the truth of a mortals existence becomes so fixated on the maddening existential horror that such a creature must experience that it dies of fright, or else becomes a spirit of light and purity, devoted to protecting our world from the horrors of their former home.

I take a riff on Warhammer's demons for my d&d setting. One wavelength of magical energy is particularly psychically sensitive. Beliefs or emotions stain the local "demonstuff" layer, and if sufficiently strong, will form a "clot". The energy then crystalizes into a "demonstone", the heart of the newborn Demon. Demons constitute nearly all supernatural entities; gods, demons, fairies, major elementals ("raw" elementals are not demons), saints, some ghosts, and many legendary monsters.

Demons can create minor demons by breaking a shard from the mother Demonstone. These minor demons often make up the bulk of a given Demon's forces, though this can be a costly and risky process. When a minor demon is slain, it condenses into the demonstone shard that birthed it. These stones are rife with power and are the easiest and most available source of power for mortals.

I've got it down pretty simple.
There is only one being that can be compared to "Demons" or "Evil spirits", and that's The Devil.
That's it, just him.
The Devil isn't as strong as God, but at the same time it cannot be touched by it. The Devil is more similar to a force of nature. A mad, unpredictable, inherently evil force of nature.
Once The Devil sets its eyes upon you (and when doing that, it usually marks you somehow), bad things will happen. Those bad things will usually lead to the marked one's death.
Some cultists think that by making sacrifices painting symbols in blood they can attract The Devil's attention and gain some form of demonic strenght. Well, they're wrong. Those sacrifices just make The Devil more powerful, and attract his attention, resulting in the cultist dying in horrible ways.
There is no way of stopping The Devil.
It is there, and it will stay there.

Damn I'm using this

I like the D&D approach of having Chaotic, Neutral and Lawful demons, but I dislike a lot of the specifics (like demons, daemons and devils being different things). My chaotic demons are inspired by Hieronymus Bosch and Bruegel the Elder and my lawful demons are inspired by C. S. Lewis and Wayne Barlowe.

Demons are what happens when divine essence lays stagnant and festers. They often are formed in the sealed temples of forgotten gods, or the tombs of divine beings.

Human beings also contain a spark of divinity, and so if their corpses aren't properly tended to they can on rare occasions become vectors for demonic forces to grow and spread, especially in places like mass graves.

They are almost universally malicious.

A modified Planescape setting:
The souls of dead mortals cluster together with those with similar take on existence. Their shared ideas manifest as Outer Planes.
A soul takes the form of a petitioner due to memories of their life in Prime Material Plane, living an idealized (for good or ill) version of their life.
Over time the petitioner loses their ego and melds with the shared dream, becoming one with their plane.
Eventually the dreamer re-emerges with newfound understanding of the workings of the multiverse and their place in it. They rebuild their body and character, drawing from both their mortal life and cultural and universal archetypes. The result is an Outsider, a true native to of the Planes.
Both Upper and Lower planes have great interest in grooming mortals to join their ranks in time. A demon causing trouble in mortal realms is doing so partly for the evulz but also as a form of tough love.
While all demons have ruthless streaks (you don't go to Hell for unpaid parking tickets), and while most are happy to snatch low-hanging mortal souls if the opportunity arises, they do have interests beyond that, and don't necessarily go around kicking puppies without a reason. And while the Upper and Lower planes have ultimately irreconcilable differences about how the multiverse should be, there are friendships and even love crossing the border, both as leftover from mortal days and as later developments.

tl;dr humans are basically larval state of both demons and angels.

Dunno, but have a demon related image I guess

"Sapient" manifestations of malignant will which coalesce in the aether. They are the setting's equivalent of a black holes, just with 100% more knowing what it's doing, probably.

If there are demons, i think its the little demons who have the most power. The demon of headaches and lost socks has way more power than the greater demons, because they cause way more suffering overall.

a million headaches is worth a thousand times more than one piddly murder.

I suppose if there were any one demon I struggled the most with, it would be miserlyness. It makes me angst for things I can not have and puts me in a mood to do things I wouldn't otherwise do.

Pure Evil.

The Devils live in a Matrioskha-like world with ten layers. The surface is Limbo, a wasteland of hungry monsters. Each layer is more powerful and organized. The Red Star in the center is the god which made this Hell.

The demons are different. For example, the three Baba Yagas were witches which either made pacts or were corrupted by them. They are only servants of the Walking Houses.

Angels and demons are all the same shit.

The only difference is the mortal perspective.

Just imagine an entity that guards a door in a dungeon, and it only lets people that say the proper words through.

To those that are killed or forced to flee, it is a demon.
To those that may enter and recieve aid, it is an angel.

I do Demon as a catch-all term for a denizen of a circle of Hell, but usually refers to a being from circles 1-6, Limbo, and would be considered Neutral. Their Demon Lords represent stuff like Order (though he's special), Death, Knowledge, etc.
Devils are demons from the other seven circles and represent the 7 deadly sins. They are directly opposed by the seven archangels.

Well currently in the timeline the Lord of Pride got purged by angels and Order gave his seat to a Limbo Demon to make the Circle of Knowledge. the Devils were getting fat and lazy and needed motivation to actually become equals to their opponents again and restore Order themselves.

Demons have a deep red and grey color scheme. Devils have crimson and black. they can be other colors themselves but their fire is always gray/black. They're usually mammals or reptiles mixed though Devils can also be insectile. Angels are often Avian and I totally stole their look from Darksiders.

So, metaphysically, emotions have a sort of gravity to them - Misery loves company and all that.

The major difference in the physics of the realm is that this gravity is larger, and the emotion can exist outside of the active support system of a human mind, floating about in a sort of quantum state. If enough of one raw emotion exists in one area, it congeals, eventually becoming thick enough to start pulling together mass.

This is where Demons come from.

Demons can be formed of positive emotions, though that doesn't really change the fact that you don't want them around. Usually though, they form because someone has been going through a world-shattering rage or psyche-breaking depression, so they tend to show up near evil folk, and so the uneducated masses call them demons and start witchhunts if one is found.

Demons simply display an excess of the emotions that created them. Demons of Wroth destroy everything they can get their hands on, Demons of greed gather as much shit as possible, Demons of Lust are giant rape machines, Demons of Love try to "spread" love by urging the people around them to be as naive as they are, and so on.

Since demons are formed from emotion and only gather matter later on, they don't have a set physical form or shape to be recognized by. Recorded instances have been talking trees, unicorns, fluffy-wing angels, dragons of dirt, a bipedal scrap-metal coyote, and in one hilarious instance, a particularly angry set of chinaware. The durability of the demon is dually dependent on the material it had at hand when creating a body, and the strength of the emotion that made it. If you see one, contact your local exterminator immediately, and stay away from any inflicted area until the authorities give the all clear!

I picture jinns as unable to interact with the physical world too, as they are clusters of information organized by (most of the time malicious) intentions. They can only catch information and communicate it between them (twisting it and distorting it in the process), and sometimes to humans by willing/"screaming" it hard enough, though mortals only perceive it as a whisper or a mere suggestion, unaware of its true origin. Humans can train to tune their spirit to a jinn's "frequency", but to do so would mean corrupting your soul to the point it starts resembling that of a jinn. This involves shady stuff like torturing living things, deceiving people, spreading lies, etc. Since Jinns exist on a plane of information they can "steal" information like physicals beings can steal matter. By spreading lies and luring others, they become more corrupt, more jinn-like, which gives them access to more information. This way, they prevent other from knowing it, and they gain the ability to have more from themselves. I guess that would be the equivalent of "stealing".
Their only power against mortals is influence and suggestion, they can't corrupt anyone by themselves, only trick them into willingly corrupting their own souls.

I like your idea of fiends.
>repented demons
noice

Well jinn aren't pure evil for me and wouldn't torture someone just for the fun of it. I tried to base them off of Arabic Jinn as much as possible. They have free will, freedom of worship (arabs said jinn could be jewish or christian) and have a wide range of motives and personalities.

So they're more like a shifty gypsy merchant than anything, one who's going to give you a fixed deal so it gets all your money and the thing it sold you back. They can appear in the physical world if they wish (they have the most "presence" compared to imps and other lesser demons) but can still become unseen if it suits them.

Angels, ghosts, and demons are the souls of humans that have been reincarnated on the same plane so many times that they cannot make themselves a body, so one is made for them by the goddess of the plane. Each one considers themselves to be moderate (like humans view themselves as the most balanced race), so angels view humans & ghosts as demons and themselves as ghosts, while demon view humans and ghosts as angels and themselves as ghosts. Demons are special, because, while the joy of heaven and the pain of hell *should* wear off after a few lifetimes, the goddess of hell created demons to be everlastingly tortured and tricked into thinking humans and angels cause this pain (she was originally the goddess of earth, but she was an asshole).

>Arabic Jinn
There are 2 version of those. Pre-islamic Jinn are said to hold mysterious powers, kinda like Genie and Jafar in Aladdin. They can either possess* someone and make his life a living hell or if subdued or befriend, they can make you rich and powerful.
Islamic doctrine on the other hand insists that "they exist only in the unseen world" (الغَيْبُ) as you mentioned, and can only influence or lure people into disbelief and injustice... if they happen to be evil, that is.

The reason I want them to be evil is because it makes them a more organic feature of the universe, otherwise they wouldn't have any reason to interact with mortals. They'd just be mysterious and withdrawn like fairies.
Giving them the ability to manifest on the physical plane is a bit generic, and overlaps with other interesting physical creatures. In my homebrew Jinns try to whisper things to pious priests and monks to bait them out of their prayer/meditation/communion, this puts players in a position where they have to chose between listening in and possibly angering their gods or letting a potentially crucial information slip. I think it's a good way of portraying how risky and consequential it is to strike a deal with the devil, especially when you add the corruption stuff.

*I don't think it was originally part of the Arabian folklore that djinn had the ability to possess people, although this belief has become mainstream today among Arabs, but that's another topic entirely.