Res Arcana: The demons have widely varying appearances, such as an imp with fins on its head, a possessed sword, a cursed dwarf king, a shadowy figure and a Ghost Ship. They share a Dark Is Evil theme, with the exception of the Homunculus (which is merely creepy and helpful for summoning other demons).
Demons are generally somewhat expensive to summon, powerful, and potentially a hazard to their summoner. (A classic example is the Lord of the Pit, which basically turns on its controller if not regularly fed other creatures.) Most of them fall into the purview of Black on the game's color wheel, and are usually portrayed as living embodiment of black mana; this prevents them from becoming Planeswalkers, as they cannot leave the planes whose magic they embody.
Most demons are the bat-winged, clawed, fanged, gigantic monster kind. However, one early demon is the flightless, half-fungus Mold Demon.
On Kamigawa, Oni are a particularly evil type of demon that make deals with ogres. Kamigawa is one of the few blocks to have demons without a trace of black mana in them — there are two oni that are mono-red.
In Innistrad, demons cannot be truly destroyed, only sealed away, and their inhuman desires and urges take physical form as little devils that spawn from them and wreak havoc. Although demons are Black-aligned, devils are Red-aligned. Griselbrand finds out that demons aren't so indestructible when up against a Planeswalker.
Demons on Kaladesh are long extinct. However, skilled artificers can use something called the Dark Schematic to create artificial demons that hunger for Aether.
Amonkhet is home to at least three different kinds of demons: the lanky, intelligent and frankly terrifying winged beings that dwell in the Ifnir Ruins within the plane's endless desert, the less intelligent crocodilian ammits that serve the goddess Bontu, and a strange-looking and unremarked-upon scorpion demon known as a Soulstinger.
Ixalani demons (who have appeared in card art, but never gotten a card of their own) are eyeless, bat-like beings heavily associated with the god of night Aclazotz.
The only known demon on the plane of Ikoria is a massive, demonic sea monster.
The demons of Kaldheim are essentially demonic Vikings, and were trapped by the gods in the hellish realm of Immersturm, where their violent natures drive them to war ceaselessly with one another. They reproduce by killing people — every time a demon kills a non-demon, its victim's blood goes to a volcano in Immersturm filled with a lake of gore; once the lake rises enough, a new demon crawls out. They're divided between demons tied to black mana, who are tyrants and conquerors wanting to spread a realm of fear and terror over Kaldheim, and ones tied to Red mana, who just really like killing, maiming and burning things.
The daemogoths of Arcavios are multi-armed, multi-eyed, wingless, and seemingly rather proactive in their search for dupes to make deals with. They are associated with Witherbloom College and as such are associated with green as well as black mana.
In New Capenna, demons now span all five colors of mana, the head of each crime syndicate being ruled by a demonic kingpin. Two of these factions have demons not aligned with Black at all: the Green-aligned Cabaretti are socialites throwing parties you can never leave, while the White-aligned Brokers are lawyers specialised in Deal with the Devil.
There are also the Red-aligned devils, which appear much more rarely, mostly resemble small red imps, and are chaotically violent and mischievous beings whose potential to do harm is mostly limited by their lesser size and power. The devils of Innistrad are said to be the destructive urges of demons made manifest, and mostly serve their mightier masters. On New Capenna, devils appear as ordinary citizens in a role similar to goblins on other urbanized worlds.
The Dark Eye: Multiple types of demon exist, each in the service of a specific archdemon, which are often summoned by mages to serve various purposes. They cannot enter the material world of their own will, and must rely on summoners to do so. Specific demon breeds each have a base shape most members of that kind of demon take, but variety is common both among individuals and broader regions — demons that take after animals, for instance, tend to resemble the closest local alternative if their associated creature isn't native to the region where they're summoned.
Difarim are difficult to observe, because they are constantly in motion and usually perceived only as a moving blur, but are thought to look something like an awful rodent. They are often summoned to serve as messengers or spies, or as speedy, agile fighters with which to wear down enemies.
Karakilim are flying serpents with batlike wings and a single horn somewhere on their bodies. They are typically summoned to serve as flying mounts or aerial troops.
Karmanthi resemble ice-white wolves with frozen spikes jutting from their elbows and ankles. They possess freezing bites and supernaturally frightening howls, live purely for the thrill of the hunt and of tearing prey to pieces, and are typically summoned to serve as hunters.
Laaranim are coal-skinned, warty, neckless, perpetually drooling horrors who can take the shape of seductive humanoids. They are usually summoned as spies and infiltrators, or to seduce, corrupt or enslave political enemies.
Shruufya are colossal, vaguely avian, two-limbed creatures with five-eyed beaked heads crowned by four horns and five tentacles. They are violent creatures that live to kill and maim, and are thus often summoned to serve as wartime juggernauts, but are intelligent and patient enough to also serve as efficient guard monsters.
Zantim resemble nine-foot-tall upright tigers with slimy fur, numerous open wounds, and armored draconic tails tipped with sharp bone blades. Some resemble other big cats instead, or more rarely take the form of other predators such as canines. They're brutal, violent monsters, and have little use outside of the battlefield.
Dungeons & Dragons has plenty of evil creatures from hellish dimensions, which led to a moral panic over alleged links to Satanism during the 1980s, resulting in the game shying away from the words "demon," "daemon" or "devil" for a time. The catch-all term "fiend" can be used to describe the inhabitants of the Lower Planes, all of which are to some extent Made of Evil (as well as potentially Law or Chaos, depending on alignment). This makes fiends inherently magical and difficult to harm with conventional weapons, and depending on where they come from they may be resistant or outright immune to specific kinds of energy damage, but they're also subject to spells like detect evil, smite evil, or protection from evil. Many fiends are innate spellcasters, and several have the ability to potentially summon more of their kind (who, thankfully, cannot in turn summon further reinforcements), while some also pick up the trick of possessing creatures or objects. Some mortals summon fiends to command in battle, consult for information, or otherwise bargain with, though such activity is dangerous and opposed by goodly churches. If slain on another plane, a fiend will regenerate back on their home turf after a time, hence why summoned fiends fight without fear or self-preservation. Though fiends are inherently hostile toward angels and other good outsiders, the Planescape campaign setting explains that fiends are also involved in the Blood War, an eternal conflict of Evil Versus Evil over which infernal faction will wage an apocalyptic final war against the Upper Planes for the fate of creation. As for specific types of fiend:
invoked Demons (properly tanar'ri) are Chaotic Evil fiends spawned by the Infinite Layers of the Abyss. They are the most monstrous, numerous and bloodthirsty of the fiends, and seek to destroy all that is good in the multiverse. Fortunately, they are also the least organized and most fractious, so the various Abyssal demon princes spend as much energy fighting each other as they do invading other planes. Demons can be "promoted" into a more powerful form (or "demoted" to a weaker one) by a patron like a demon prince, or this may occur spontaneously at the whims of the Abyss itself. In the original D&D, demons were the only type of fiend, and their strongest were explicitly balrogs until the Tolkien estate complained, prompting a rename to balor. Other demon types include vrocks, literal feathered fiends that release damaging spore clouds in combat, mariliths, multi-armedsnake women who serve as demonic generals, and glabrezus, dog-headed, crab-clawed brutes who feature prominently in drow graduation "parties". They are traditionally weak to Cold Iron.
invoked Obyriths are primeval fiends who ruled the Abyss before life arose on the Material Plane, and as such are so horrifyingly alien in appearance that just being close to one is in itself a Brown Note. They were overthrown by their tanar'ri slaves eons ago, but some obyriths remain, including several demon lords. The loumara on the other hand are a comparatively young fiendish race spawned from the Abyssal layer of the Dreaming Gulf, and are characterized by lacking physical bodies and instead sowing chaos by possessing creatures and objects.
invoked Devils (properly baatezu) are Lawful Evil fiends that rule the Nine Hells of Baator. These are the fiends most likely to tempt mortals into a Deal with the Devil, since every damned soul that arrives in Baator is tortured and twisted until it becomes a mindless Lemure instinctively seeking to climb the Hellish hiearchy. As such, devils are out to bring the multiverse under their tyrannical rule and "save" it from the demons' attempt to destroy it. Despite their adherence to law and vast infernal bureaucracy, devils have no shortage of scheming and backstabbing, and Baator has seen several unsuccessful attempts by its various archdukes and archduchesses to unseat Asmodeus, Lord of Hell. Most Fallen Angels seem to end up in Hell, and according to legend, Asmodeus and other first-generation devils were originally angels who became corrupted from fighting against demons when the cosmos was young. Devils are traditionally weak to silvered weapons, and tend to look the most human of the fiends, which is not to say that the sight of a towering, fanged, scaled, winged pit fiend is in any way reassuring. Other baatezu include imps, who often serve as familiars and advisors to mortals, merregons, maskeddevilish legionnaires, erinyes, femaleformer angels turned infernal shock troops, and narzugons, nightmare-riding knights with lances of hellfire.
It is rumored that the Nine Hells were once inhabited by bizarre creatures known only as Ancient Baatorians, which are said to have consumed light and exhaled "anti-light", before ascending into forms of pure malicious thought well before the baatezu arrived. Traces of them can still be found in the form of unearthly sounds echoing from one cavern beneath the layer of Malbolge, or strange shapes frozen within the deepest of Cania's glaciers. Devils who investigate such anomalies tend not to return.
invoked Daemons, more commonly known as yugoloths, are Neutral Evil fiends originally from the Gray Waste of Hades, but now mostly live in the Bleak Eternity of Gehenna. Supremely self-interested, mercenary and manipulative, the yugoloths play both sides of the Blood War against each other for their own benefit. They can be reasoned with but can't be trusted to keep their word, and any contract they sign with an employer will have the stipulation that the yugoloth is free to change sides if it receives a better offer. Like demons and devils, yugoloths have a chance to summon more of their kind in an emergency, but unlike the other fiends, these "reinforcements" are not automatically allies of the summoner. Notable types of yugoloths include canoloths, eyeless guard dogs with long, spiked tongues, merrenoloths, who ply the River Styx as freelance ferrymen, arcanoloths, jackel-headed mages and manipulators, and ultroloths, mercenary captains whose faces are blank save for their hypnotic eyes.
Baernaloths are thought to be the vile yet emotionless Precursors of the entire fiendish race, the progenitors of the yugoloths, demons and devils who dispassionately spread evil across early creation. This theory enjoys few adherents outside of Gehenna, naturally.
The baatezu, tanar'ri and yugoloths are only the major types of fiend, there's no shortage of other evil outsiders that don't belong to those hierarchies: the demodands/gehereleths that act as both prisoners and jailors of the Tarterian Depths of Carceri, tiger-headed rakshasas, hell hounds, soul-eating barghests, oni, Succubi and Incubi, and so on. Fiends are capable of reproducing with mortals, resulting in winged half-fiends called cambions that are typically just as nasty as their infernal parent. Fiend-descended mortals are known as tieflings, and usually have infernal features such as glowing eyes, horned heads, cloven feet, and sinuous tails that make them subject to suspicion and prejudice, even if the tiefling isn't of evil alignment. Tieflings have been a standard playable race since the game's 4th Edition, in which they were the scions of a fallen empire that trafficked with devils.
Basic D&D, which had a different and in some respects more complex cosmology from the Advanced version, cast demons as not-quite-immortal creatures aligned with the Sphere of Entropy and had no separate devils as such. It did, however, have the "diaboli," sinister-looking humanoid creatures native to the Dimension of Nightmares who in a bit of a twist (and unlike some other monsters from the same dimension) were very much presented as a case of Dark Is Not Evil.
Eberron: Demons and devils are not a huge part of the setting. The concept of the dimension-spanning Blood War is limited to a single plane, Shavarath, where the demons, devils, and angels are all locked in constant combat. Yugoloths still presumably exist in Eberron's cosmology but are given little attention. The more important fiends are the rakshasas, who are actually native inhabitants of the world rather than extraplanar beings, and they ruled the planet for millions of years before being ousted from power by the dragons. The setting also includes two other broad categories of fiend, daelkyr (Humanoid Abominations who are thematically closer to Cthulhu and co. than traditional demons) and quori (nightmare spirits who seek to preserve their own reality by controlling all mortal dreams, and already rule one continent through the original Path of Inspiration) which are not technically demons, but might be colloquially referred to as such.
Dragonlance: Demons, devils, and daemons exist, but they are all subservient to the gods of evil. Since the evil gods seem to prefer using their mortal servants to carry out any tasks they want done, the three fiendish races are rarely seen.
Ravenloft: Demons (and fiends in general) are rare and mysterious beings, which enter the setting by transposing their bodies with those of morally corrupted human beings. Once in the Land of Mists, they are unable to return home; many acquire additional powers by performing rituals in various domains. Demons are able to cross sealed domain borders freely, as their vast evil cancels out the influence of darklords in a small area around them.
All true demons are a type of spirit that is a part of a Yozi — Eldritch Abominations/dethroned Creators of the Universe. Each of the twenty-three Yozis has multiple souls with minds and wills of their own — at least twelve, but the most powerful among them, Malfeas the Demon City, has twenty-three. These are called Third Circle Demons. In turn, each Third Circle Demon itself has seven souls, which are called Second Circle Demons. The Second Circle Demons can create new species of demons, which are known as First Circle Demons. The First Circle demons can range from twenty-four fingered harpists who use time as a harp to beetles that swim through flesh and eat poison. Oh, and shape-shifting prostitutes are in there as well (they don't want your soul either, just some flesh to make a baby with way too many parents). Oh, and the major religion of the setting considers summoning them OK as long as you know what you're doing (and are a Dragon-Blooded, mortals shouldn't reach above their station, which is genuinely good advice on demon-summoning in-game). In infernal society, Third Circle Demons are the unquestionable nobility, Second Circle Demons are the equivalent of minor nobles and free landholders, and First Circle Demons are slaves, dispossessed peasants and disposable minions.
The Infernal Exalted come from Solar Exalted shards warped by the aforementioned Yozis. While the Infernals start out as humans given great power with a few demonic touches and a First Circle demon's voice in their heads, they ultimately develop into something closer to a Yozi than a human. And they don't have to stay under their Yozi patron's control, either.
Fabula Ultima: Demons are a category of monster. They are described as "incarnations of legends and beliefs" and are stated tp not be truly alive despite possessing a corporeal form. The demons presented in the core rulebook's bestiary include cowardly imps, fearsome Shadow Howlers, and cunning Echidnas.
Gamma World demons are an incredibly recent addition (As in, this current edition is the first to have them) to the game, basically being other-dimensional beings that can only appear in the world with the right mathematical equations. Some of the demons include a Creepy Child, a Sinister Minister, a demonic police officer that is an obvious homage to O Brother, Where Art Thou?, and a gigantic Poisonous Demon.
GURPS, being designed as a Universal System, can support a wide range of demons and devils to suit different settings. For example:
GURPS Dungeon Fantasy takes a distinctly modular approach to the demons which serve mostly as powerful monsters to kill, perhaps after being summoned by evil wizards or demonologists.
GURPS Thaumatology: Alchemical Baroque: Devils are mean and malicious spirits from Hell who sometimes travel to the material world, where they manifest as "bright red humanoid figures, somewhere between six inches and two feet tall, with wings, short horns, and chittering voices", which again demonstrate various supernatural powers, and can be clever tempters, despite the fact that everyone knows how nasty they are.
GURPS Thaumatology: Age of Gold: Demons tend to follow local legends and popular beliefs very closely, in behavior and appearance — which makes them extremely dangerous but sometimes also faintly ridiculous. One theory is that they are projections of the human id, which would explain why they are so heavily shaped by the popular imagination.
Infernum: Demons are biological weapons created by Fallen Angels by crossbreeding themselves with Spawn (sort of mindless, soulless, prototypical lifeforms), then established into the nine Breeds (Artificer, Beast, Deceiver, Fiend, Hulk, Imp, Malcubus, Stalker, Slaver) by a long regime of surgery, crossbreeding, incest and black magic. Realising they were expendable in the eyes of their "fathers" they rebelled, ate all of them they could catch, chased the others clear out of hell, then took the place over. They torture souls to extract a substance called 'iliaster', which is literally meat, drink and fuel to them. And these are your player characters. Insofar as nomenclature goes, "demon" is the name of the race, "devil" is a title given to demons that have performed a notable and impressive feat and "daemon" is the lowest of the noble ranks, awarded to demons that display true mastery in some art or skill.
In Nomine has an incredible assortment of demons. All they have in common is that they're Fallen Angels, and every demon is a perverted version of the angel they used to be. The exceptions are the Lilim, who are all the children of Lilith and were never angels. While all (non-Lilim) demonic Bands are indeed perverted/corrupted versions of a Choir of angels, the vast majority of "young" demons have been created in Hell. The difference in outlook of a "Hellspawn" (who never saw Heaven and just knows the propaganda of the Demon Princes) and the actual, older Fallen Angels (who do, often painfully, remember their time in Heaven) is pointed out in one of the player character sourcebooks as important to keep in mind when roleplaying a demon. Still, even Hellspawn who never were angels in the first place can achieve redemption and become one of the Choir that corresponds to their Band.
Balseraphs: Master liars — their lies are so effective that they think that their lies are the truth — and the most selfish things in Hell; fallen Seraphim. Their celestial forms are immense snakes with bat wings instead of the feathered ones of Seraphim, and tend to be blood-red.
Djinn: Stalkers who do everything in their power to deny that they care about anything apart from themselves; fallen Cherubim. Their celestial forms are blends of various beasts.
Calabim: Blood Knight demons who use both their entropic powers and old-fashioned physical violence in their role as the blunt instruments of Hell; fallen Ofanim. Their celestial forms look like stereotypical demons (red skin, bat wings, and the like) and sometimes look visibly deformed as a result of the Discord they naturally possess.
Habbalah: Emotional manipulators and tormentors. Insane even by demon standards (they think that they're angels doing God's work where it's needed the most — in Hell: for them, "doing God's work" means "punishing" anyone they deem to be weak); fallen Elohim. Their celestial forms are covered with scars, piercings, tattoos, and other painful-looking mutilations.
Lilim: Demons who can give you anything you need... but you'll have to pay for it eventually. All Lilim are created in Hell by Lilith, but some occasionally join Heaven as Bright Lilim, and others manage to clear their debts and go Free. Their celestial forms appear as green-skinned humanoids with small horns and bat wings.
Shedim: Really really creepy puppeteers of men who corrupt their victims, degrading their moral standards over time and convincing them that their demonically implanted suggestions were the victim's own ideas; fallen Kyriotates. Their celestial forms are clouds of monstrous and unnatural-looking body parts.
Impudites: Charmers who can steal your heart — and bits of your soul, too; fallen Mercurians. They are not allowed to kill a human, though. (It's such a waste of good food.) Their celestial forms resemble the stereotypical demon, with bat wings, horns, and so forth.
Pachadim, described in The Marches: moles in human society who spend decades carefully ruining lives, slowly working on manipulating and expanding on the fears and insecurities of humans and sending the resulting wrecks out into the world to do the same to others; fallen Menunim. In celestial form, they resemble obese, diseased, pustule-ridden humans.
Additionally, Hell is populated by teeming hordes of weaker spirits collectively referred to as imps, snots or gremlins. In Hell, they serve as a combination of disposable flunkies and sapient vermin; on Earth, they're usually sent to perform petty but frustrating sabotage against humans. If these critters accumulate enough spiritual growth, they can eventually fledge into true demons of any Band; the Princes usually mass-produce them and leave to fend for themselves, reasoning that the ones that live long enough to mature into demons will be the ones with the most potential as actual servants.
Middle-Earth Role Playing: Demons, or Raukar, are the various spirits corrupted by Morgoth. They're trapped in monstrous physical forms that mimic the aspect of nature they once oversaw, which they can no longer change or abandon for an incorporeal state, and will perish if these are destroyed. They cannot reproduce — no spirit can. Multiple kinds exist:
Balrogs are fallen Maiar of flame, and the mightiest of all demons. They are deadly in combat and can seize and enfeeble the minds of mortals, and while their mindsets are largely impossible for basically decent people to fathom it's known that they are fiercely loyal to Morgoth and have little respect for any creature save each other and dragons. Their magic is mostly concerned with fire, destruction, and summoning other evil beings.
Black Demons are a generic collection of weaker spirits in Morgoth's thrall. Their forms are immensely varied and unique, although many are elemental in nature, and they can sometimes be persuaded or forced to obey the commands of mortal summoners.
Lassaraukar, or leaf-demons, are corrupted wood spirits in the form of tall, featureless men with thick green skin and a natural "veil" of skin hanging from their temples and hiding their faces. They are insanely fast and agile beings, delight in spreading death and terror, and fight with stone disks coated in their own venomous blood.
Vampires were once patron spirits of bats, but under Morgoth's influence have become monstrous, evil creatures resembling immense bats with ugly human heads and grasping talons. They also possess magical cloaks that can be used to take the shapes of others. Morgoth used them chiefly as messengers and spies.
Nobilis features both Devils and Demons. Devils are Fallen Angels (the motive for their Fall differs between edition: in first and second editions, they held corruption to be the highest principle, the foundation of existence, while in third edition, they fell because they believed everything, no matter how evil, deserved love); Demons are the beings who have populated Hell since before Lucifer's Rebellion, and are on a much smaller (spiritual) scale.
It follows the example set by 2nd and 3rd Edition D&D, minus the Blood War. It defines the actions of the three fiendish races by their motivations: the Lawful Evil Devils want to corrupt the minds of mortals and bring all the universe under Hell's iron law, the Chaotic Evil Demons want to corrupt the body and make all the universe their eternal playground, and the Neutral Evil Daemons simply want to kill and strip the entire universe of all life and thought. Devils want dominion, Demons want destruction, and Daemons want mass extinction.
There are also several lesser races of fiends in Pathfinder. The Asura are Lawful Evil, inspired by Hindu mythology, and are created by the mistakes of gods and enjoy corrupting their servants and destroying their works. The Divs are Neutral Evil corrupted and fallen genies, who enjoy destroying the great works of mortals. The Oni are fallen kami who adopt physical forms based on distorted versions of various humanoid races. The Qlippoths inhabited the Abyss before demons arose and are incomprehensible and nightmarish. The Kytons are a race of torturers based on the cenobites from Hellraiser. The Sahkils are fallen Psychopomps who abandoned their divine charge to become incarnations of fear. The Demodands resemble distorted caricatures of humanity, and were created by the thanatotic titans as servants and worshippers. The Dorvae are cruel reptilian fiends who each believe they are the unquestioned master of existence. The Rakshasa are Hindu-inspired, and a tyrannical race born from the reincarnation of mortal conquerers and tyrants.
Reign: Demons — although exceptionally powerful magical beings — are of decidedly natural origin, hatching from eggs that form in the earth of the game world. They also get really freaky as they get more powerful, to the point that the most powerful can usually break off parts of themselves to act as independent agents, and are statted with the system the game uses for kingdoms and organizations, rather than characters. Also, since they hatch from eggs in the ground, the world's giant fleas made of earthen materials which suck minerals out of the earth tend to be full of them.
Rifts: Demons come from Hades, Deevils come from Dyval. They don't like each other very much. There are also the Chaos Demons, the Archaic Demons of Russia, the Oni of Japan, and the Infernals of China, who are all separate groups. Technically, only denizens of Hades are "true" Demons, but in practice the term is usually applied to any supernaturally powerful intelligent species (even Deevils, who tend to become homicidal when confused for Demons).
The Splinter: Daemons are more similar to malevolent elementals than traditional demons. There is also the fact that they're part of a massive VR simulation that somehow became real.
Talislanta: Demons are chaotic beings associated with various elements (e.g. Aqua Demons, Frost Demons, Pyro Demons), while Devils have a complex hierarchy and are governed by a strict set of laws and customs.
Unknown Armies: Demons are treated more like ghosts are elsewhere. They are uniquely obsessed souls who were incapable of letting go of the mortal world, and as a result their personality rots away until the obsession is all that's left. In order to fulfill the obsession (which can be anything from finishing a novel to eating painfully spicy Mexican food), they have to possess humans. They can be summoned by magick users, but while they do usually know about the occult underground to some extent, they can and will lie as much as possible in an attempt to get a body to ride. Worse, someone with a high Soul can unwittingly allow a demon to possess them any time they play with an Ouija board or even pray. Demons aren't evil, exactly, but because of their focus on fulfilling their obsession, they will only show compassion if it's convenient for their goals. They're amoral, but not necessarily immoral — though given the need for possessing humans, morality falls by the wayside fairly quickly.
Demons can also possess animals, but by cramming themselves into such simple beings, they lobotomize themselves in the process. Eventually the spirits of the demon and the animal intertwine, and this fused soul can then possess a human and create a lycanthrope. Lycanthropes are a three-way struggle between the animal, the demon, and the poor blighted human host. When the animal is in charge, the person retroactively becomes that animal, and further was always that animal. The human is erased from existence entirely, but in a roughshod fashion that reveals that something's wrong if you know where to look. When the human or demon regain control, the human always existed, and the animal never did. This can lead to such oddities as someone seeing a lycanthrope crow, which becomes a human again, which leads the witness to believe that they saw a flying person but didn't think anything of it.
The kicker is, because the adept schools of magic require a near-delusional obsession with a paradoxical worldview, adepts are far more likely to become demons than anyone else. So if you're an adept — and you probably are, as it's the fastest path to power in the game — you might have a rather interesting afterlife to look forward to.
Demon: The Fallen: Demons are angels who were stripped of corporeal form and cast into the Abyss; a recent clusterfuck in the spirit world freed most of them, though some of them were summoned to Earth by mortal sorcerers, bound to inanimate objects, and eventually became Eldritch Abominations. A demon typically needs a soulless body (e.g., someone who's brain dead, a severely catatonic mental patient, a recent murder or suicide) to retain a presence on Earth; strangely, this is actually a humanizing experience for many demons, as they get to experience what humanity's like after millennia of abuse. They still retain many of the powers they had as angels (which range from controlling flame to reshaping flesh to overseeing the dead), but can willingly tinge these powers with "Torment," which makes them more destructive and more likely to screw with humans.
Werewolf: The Apocalypse: The Maeljin Incarna are high-ranking servants of the Triatic Wyrm who reside in Malfeas. The each serve one of the Urge Wyrms by spreading corruption among humans. Banes are minor spirits that serve the Triatic Wyrm and can possess living beings, thereby transforming them into fomori.
New World of Darkness has so many examples that Demon: The Descent has to pause and say that the Unchained aren't necessarily related to the myriad of other things that could be called "demons" in the setting, and that anyone encountering a "demon" in one of the other game lines may have to do some digging to figure out just what they're dealing with. A few examples:
Demon: The Descent: God is a vast clockwork machine who manipulates reality according to its own hidden designs, angels are its constructed servants sent to interact with the world on a human scale, and demons are therefore the angels who gained enough self-awareness to deviate from their programming and seek individuality. Rather than trying to escape Hell, these demons are actually trying to create Hell, although their definitions of what Hell could be range from "destroying the God-Machine", "repairing the God-Machine", or even "escaping to a dimension where there is no God-Machine".
Mage: The Awakening has Goetic demons, created from the mage's own subconscious desires and inclination toward sin given spiritual form and manifested in the real world, and the demons of Pandemonium, natives of a Supernal Realm entirely unrelated to Hell.
Werewolf: The Forsaken reverse-ports the Maeljin from Apocalypse, making them the lords of corrupted spirits that exist only to selfishly perpetuate vices. Sure, spirits of lust, greed, and violence already exist, and feed on and may perpetuate these concepts, but the servants of the Maeljin do so by corroding people and places, making sin more likely. They tend to gather at Wounds, places of tainted Essence that emit corrupted resonance.
Hunter: The Vigil has the Lucifuge, a Conspiracy of hunters made up entirely of the children of Hell, setting out to combat their infernal legacy. They can summon minor imps and call on the powers of Hell. Because a lot of Hunter is based around mortals with guns and a few tricks who don't know the full supernatural landscape, the game is very ambiguous as to what kind of demon a hunter may encounter on any given day, and is also ambiguous as to what kind of "demons" birthed the Lucifuge (though Inferno is probably a good guess).
World Of Darkness Inferno: Demons are embodiments of sin, looking to spread the sin they identify with on Earth to increase their power. Some demons can possess humans, giving them access to special abilities deriving from their iconic sin, both literally and thematically. Demons dwell in Hell, a realm of sin incarnate; drawing in elements from Mage, it is painted as one of the Lower Depths, a realm that lacks at least one of the ten Arcana that make up all existence and which is thus anathema to life. The ultimate origin of demons is unknown; some demons are ghosts and spirits who fell to corruption, but not all.
Warhammer 40,000: The Warp is the source of all Psychic Powers in the setting, functions as the universe's "Hyperspace", and is where mortal souls pass through after death. It's made of "psychic energy" affected by, and possibly created from, the emotions of sapient life, and eventually particularly strong emotions gave birth to the Chaos Gods, along with lesser daemons who act as extensions of their corresponding god's will. Daemons of a particular god will represent various aspects of its patron.
Daemons are unstable in the Materium, and must pass through a Warp rift, be summoned via ritual, or find a host. Once they have crossed into our plane of existence, however, daemons are extremely dangerous, as they tend to be empowered by literal gods or warp reality around them. The trouble is that any psyker that has not been discovered and trained to control his abilities is susceptible to sudden, involuntary Demonic Possession.
As purely psychic beings, they are extremely vulnerable to anti-psychic weaponry and cannot even touch blanks. This has allowed some factions to develop a few specialized anti-daemon countermeasures. Such weapons are rare, however, and More Dakka is often sufficient to kill Lesser Daemons. Of course, when all else fails, there's always Exterminatus.
, which basically turns on its controller if not regularly fed other creatures.) Most of them fall into the purview of Black on the game's color wheel, and are usually portrayed as living embodiment of black mana; this prevents them from becoming Planeswalkers, as they cannot leave the planes whose magic they embody.
Lawful Evil Devils want to corrupt the minds of mortals and bring all the universe under Hell's iron law, the Chaotic Evil Demons want to corrupt the body and make all the universe their eternal playground, and the Neutral Evil Daemons simply want to kill and strip the entire universe of all life and thought. Devils want dominion, Demons want destruction, and Daemons want mass extinction.