The Adventures of Amina al-Sirafi: Raksh insists that "demon" is an erroneous human term for spirits of discord like him. His personal speciality is human ambition, not evil — though he himself disregards mortal morality and isn't beyond snacking on the occasional human heart. Though the story is based on Middle Eastern history and legend, evil spirits like Shaitan don't make an appearance.
Age of Fire has demen ("deep men") — horned, dorsal-spiked, carnivorous hominids who inhabit the more inhospitable parts of the Lower World. They're not really demons per se, but the similarities are probably intentional.
Artemis Fowl: Demons are one of eight subspecies of fairy, and are said to be descended from alien bacteria that crash-landed on earth from a meteorite. During childhood they are called imps, and when they are ready for adulthood they undergo a metamorphosis that grants them tremendous strength and physical prowess. They don't have horns (except in cover art), instead possessing ears adorned with multiple keratinized points. Though many of them cannot use magic, a rare few imps that do not metamorphose are known as warlocks, and have the potential to become the greatest mages of all fairykind.
A Most Unlikely Hero: Daemons are an alien race who draw power from some kind of inner-demon each of them have in their souls.
Astral Dawn: The Aash Ra Va function in the role of demons. The dark Earth spirits later take over that role after the Aash Ra Va leave the Nine Dark Worlds.
The Bartimaeus Trilogy: "Demons" are more like genies, who exist on a different plane and can project themselves as almost anything but have no real physical form. And they don't like to be called "demon" anyway; that's what the magicians call them.
Black Dogs: Demons are incomprehensible creatures stretching out into unknown dimensions, able to be summoned and controlled by uttering their names.
The Book of All Hours: The Unkin are humans that experienced a unique event in their life that allowed them to touch the Vellum underneath reality. In the multiverse inscribed on the surface of the Vellum, these meta-humans have long since taken up different roles, presenting themselves to mortal humans in different ways in pursuit of power.
The Book of Dragons: In "The Long Walk 2020", demons are monstrous beings with two mouths, six limbs, crystalline eyes that light up when they spot prey, and the ability to exhale clouds of deadly scalding steam from their jaws and tendrils on their heads. They're relentlessly vicious killers who attempt to destroy everything and anything they encounter, and both humanity and dragons are engaged in a desperate battle of survival against them.
Book of Imaginary Beings: According to Emanuel Swedenborg, devils are the souls of people who chose Hell rather than Heaven; they are not happy there, but would have been unhappier still in Heaven. They are hideous and malformed, with heads that are those of animals or shapeless lumps of flesh, but think themselves handsome. They are always in bitter conflict with one another, and only come together to plot and scheme against each other. Much like with angels and Heaven, devils are miniature Hells in themselves and Hell is much like a devil.
Magic Kingdom of Landover: The demons of Abbadon are exiled faerie beings who want back into the mists and are more than willing to go through Landover to do it.
The Shannara series has two kinds: the genuine Demons, who are evil Faerie creatures, sealed up beyond the Forbidding by the good Fae, and the "demons" of Morrowindl, who are really just another form of Shadowen. And then there's the Jachyras: evil Faerie lunatics so mad that even the Demons thought they needed sealing.
The Word and the Void has former humans who, having sold their souls, serve The Void for all of eternity. They have a wide range of powers, including shapeshifting, mind control, spellcasting, or just plain blending in, and seek to reduce all of creation to raw chaos.
The Calf of the November Cloud: According to the Konyek's tribe's Masai traditions, devils are half-man, half-lion supernatural beings, who masquerade as poor men in need to ensnare, kill and eat the unwary.
The Chaos Cycle: Demons are twisted spirits whose Unfinished Business has kept them trapped in the material realm. They can influence humans into killing others by preying on their emotions to make them take violent actions. They can also only be destroyed for good in the spiritual plane.
Childhood's End is an interesting example. The reason mankind "made up" demons looking like they do in mythology is because of a pre-memory of the fact that the alien race that will eventually arrive and take the children to the next stage of evolution have wings and horns.
Children of the Lamp: Demons apparently exist (one or two are encountered) and are apparently Fallen Angels, but are only vaguely described. The Evil Djinn serve as demonic stand-ins, and their three tribes (Ifrit, Shaytan, Ghul) are all named after demons or evil spirits from Arabic legends. The most powerful and evil Djinn in the world, Iblis, is even named after the Islamic Devil.
Children Of The Sun: Demons are actually air elementals (Their name for themselves is "Children of the Air"). They're not necessarily evil, but are very chaotic. They have a natural form, which can only be seen by those with magical sight, but they can assume (or be forced into by a magician) a physical form. If it's a chosen form then they can also dissolve it and their next physical form can be totally different. If forced magically into a form they are stuck in it until released or killed. If their owner has their true name then they can be returned to that form. If not then they are free.
A Chorus of Dragons: Demons are spiritual entities from the cold void of Hell, feed on souls and energy, and are evil and hateful in a way no mortal can ever match. Their energy-eating habits make them very attracted to source of heat, especially fires, and they like to set blazes everywhere they go as a result; however, they are also surrounded by chilling cold due to consuming the heat in their own vicinity. Due to losing a war with the gods, they can only enter the physical world at the summons of wizards who can then command them. They're keenly interested in changing this status quo, however, and sufficiently powerful demons who slip their bindings can set about summoning more demons and starting a destructive rampage known as a Hellmarch.
The Chronicles of Ancient Darkness: Everything has three souls: the name-soul, the clan-soul, and the world-soul. If something's souls are scattered upon death so that it loses its clan-soul then it becomes a demon: it still remembers its life, it just hates everything that lives because the light of life is blinding to demons. The other options are ghosts, who have lost their name-souls and so have no memory, and the Lost Ones, who have no world soul.
In the Colt Regan universe, Demon is a sort of catchall term for various types of extraplanar beings with whose traits vary wildly from type to type. Often containing one (or more than one) of the common traits listed above.
In Sara Douglass' The Crucible trilogy demons are the offspring of angels and mortal women who are consigned to Hell to keep them out of their fathers hair and because the angels consider procreation shameful although some have avoided this fate and seek to overthrow their fathers rule of Earth.
Dance Of The Butterfly: The supernatural enemies are called demons, devils, or, collectively, the Infernal. One of the protagonists admits he is not sure if they are literally demons, and they could just be extradimensional aliens. These "demons" do show an ability to influence humans, even going so far as possession. They are largely impervious to conventional methods of attack, and the hunters resort to special ammunition and/or the use of magick to thwart them.
In the Daniel Faust series, Demons are divided by Choirs of Sin, which influence their personality and powers. For instance, a demon from the Choir of Lust gets a corrupt cop addicted to her touch as if it was heroin, while people become so listless in the presence of a Sloth demon that they might starve to death instead of getting out of bed. Doing evil for evil's sake is a pretty low priority for demonkind, at least the ones we've met so far. They're either out to satisfy their own ambitions (Nicky and Sullivan) or they're operatives in the endless cold war between Hell's feuding courts (Caitlin and Emma). They can be polite and even friendly — as long as they're getting what they want. Stand in their way, though, and the politeness vanishes in the blink of an eye.
A Deal with a Demon: Demons in this series come in five separate species, with each having their own features:
The red skinned Bargainer Demons, which specialise in making deals with mortals for favours
Humanoid dragons which are nine feet tall and have reptilian faces and scales
The Krakens whose lower bodies are made up of tentacles.
Gargoyles whose skin is hard as rock and are greyish in colour.
Succubus/Incubus which are a form of sex demon.
In The Descent by Jeff Long, it turns out that there is a physical, real-life equivalent to Hell and demons — an underground world of tunnels and the brutish "hadals" that inhabit it. These creatures are degenerate hominids with demonic features and a taste for senseless, incredibly brutal violence. They are also led by a kind of Satan.
Demons in the Discworld series are inhabitants of Hell and are malevolent. We learn most about them in Eric, where they're said to be the same basic kind of being as Discworld gods, just ones that have found that temptations and being seen as the "adversary" are just as good for Gods Need Prayer Badly as being worshipped. (Although they're quite happy for that to happen as well.) They're shapeshifters, taking forms that vary from humanoid-but-inhuman to Eldritch Abominations, but have no actual imagination, so need to get the ideas for these forms from humans. (They should not be confused with the actualEldritch Abominations of the Dungeon Dimensions, who are "the opposite side of a coin where good and evil are one side.")
The Divine Comedy: The word "demon" is used to describe beasts and monsters who punished the damned in Hell rather than angels who are themselves punished there. As such, while Lucifer is never called a demon, Greek monsters like Charon and Cerberus alongside original creations like the winged Malebranche get the designation.
Domina: Demons are just humans who used the toy maker to add horns, fangs, and maybe a tail to their bodies. They are in fact the first of the cultures based around use of the toy maker, and were originally modeled after the Mother Monster, when the prototype toy maker was used to give her devil horns. Demons have a variety of different clans and bloodlines, including the hated succubi, but it is common for them to switch cultures. Their overriding motto is "The freedom to be whatever you wish." This means that they are on very good terms with all the other cultures; even the angels don't have a problem with them.
Dora Wilk calls all citizens of hell "hellians" and divides them into devils (fallen angels) and demons and czarts, who lived there before devils arrived and took charge. All three groups are super-strong and look like humans, although some odd horn or strangely-colored eye might come in play and demons have bristles instead of hair. Demons are stronger, have magical powers and are divided into Families of Anger, Temptation, Chaos and Beasts, and czarts are less powerful, but independent and more humanlike.
Dragaera: The negative implications of demons are completely averted in the series, which has a lot of Dark Is Not Evil present, as the term only means "someone who can be summoned" without any moral implications. Verra the Demon Goddess (the patron goddess of some of the protagonists and the mother of one of them) is called as such because she was formerly a slave to the Jenoine before taking her position. There's also a character called the Necromancer who is an undead demon summoned from a place called the World of Seven Doors, who thereafter serves Sethra Lavode and aids the protagonists. This probably also explains why a gangster in the series is known as the Demon — he's an assassin and can be "summoned" for the right price.
Death Masks and Small Favor feature the Knights of the Blackened Denarius, who are human hosts to fallen angels. Each host, after an initial temptation, took up one of the thirty titular denarii (the thirty pieces of silver given to Judas for betraying Christ). Each of the Fallen lives within its human host, who grants him or her various abilities such as Voluntary Shapeshifting. If a host is killed, the coin he or she holds comes out of the corpse, and attempts to catch another host. They are specifically known as the Fallen though. There are "Demons", used as a generic term for any being from a specific region in the Nevernever, and it's not totally clear what their relation is to Angels, fallen or otherwise.
In addition, there are a variety of super-demons known as Outsiders that come from outside reality itself. These are considered so horrible and dangerous that any wizard who summons one (or even seeks out detailed information about them) is sentenced to death. Also, one of the Senior Council members, the Gatekeeper, has it as his job description to keep the Outsiders from getting in.
The Belgariad and The Malloreon: The Demons (or, if you're a Morindim, the Devil-Spirits) are incredibly powerful beings who can be summoned by the truly reckless (or the truly stupid) to do their bidding; practitioners of this are known as magicians or wizards. The smarter magicians bind their Demons inside an illusion and protect themselves with pentagrams and other magical protections; as long as the Demon remains inside the illusion, it must do it's master's bidding, if and when it breaks out, the pentagram will (hopefully) protect it's master. The stupidest magicians just summon the Demon and try to force it into doing their bidding. They seem to come in an infinite number of forms, and have a wide variety of powers from the application of sheer brute force, to More than Mind Control, and full on Demonic Possession (of a freakin' dragon, no less). They seemingly cannot be killed, and must be sent back to their own universe; this is a pain for the heroes, especially once two contenders for the role of Big Bad start using a pair of Demon Lords as their respective Dragons; unlike normal Demons the Demon Lords are consenting to work for their masters, largely to fulfill their own agendas. Hell, their home base, is alternately described as another universe, and a "state of mind"; they are said to work for a Satanic figure known only as "The King of Hell," who seeks to become ruler of the universe. It's worth noting that summoning a Demon never ends well; eventually it gets away and either eats you, or (in the case of the two Demon Lords) drags your soul into Hell and feeds upon it for the rest of eternity.
Fancy Apartments: The main villain is a dark spirit named Vrotheus the Vile. Dangerous, powerful, evil, and with a great fondness for hexes and curses. Oh, almost forgot to mention: is also a large ham.
Fantastic Beasts and Where to Find Them describes some magical beasts as demons. All they have in common is that they're small, vaguely malevolent, and rather pathetic.
Felix Castor has demons of many stripes, ranging from the old standards (Asmodeus, Moloch, succubi like Juliet) to more mindless, rampaging things (The Naming of the Beasts sees something Juliet compares to a herd animal of Hell possessing a gymnasium). Like ghosts, they can be called up and exorcised by any number of means. And on rare occasions, it's possible for a human ghost to metamorphosize into a demon.
Forest Kingdom: In book 1 (Blue Moon Rising), the Darkwood's demons are mindless Mix And Match Creatures with no drives except to cause pain. Except for the Demon Prince, who isn't mindless. Book 4 (Beyond the Blue Moon) eventually reveals they're actually humans transformed by the Demon Prince's power.
George and Azazel: The titular Azazel is just out to help people... but his poor understanding of humanity and rather incompetent human intermediary tend to cause it to fail hilariously. Though he may actually just be a Sufficiently Advanced Alien. Also, he's only two centimeters tall, and George may be making him up.
Grounded for All Eternity: Demons are human souls who've been corrupted by their own evil into something new, as is implied to be what's happening with Parris.
Douglas Hill's young-adult novels Blade of the Poisoner and Master of Fiends featured demons that resembled everything from giant vultures to grotesque crystalline golems to Eldritch Abominations. They were all hard to kill to some degree or another, and all uniformly nasty.
Demons are a key feature of Mitchell Hogan's dark fantasy universe of The Tainted Cabal and The Infernal Guardian. They are otherworldly beings who inhabit a number of abyssal realms and can be summoned and commanded by human sorcerers using their true names (a practice for which demons despise mortals and commonly refer to humans as "the slavers"). They are organized into various "orders" based on power level, and can gain power by killing other demons and absorbing their essence. Lower-order demons are essentially berserk animals totally ruled by hunger and cruelty; higher-order demons are much more intelligent and, though they are typically ruthless, predatory and with morality of a rather blue-and-orange nature, can potentially be reasoned with. Demon lords are especially powerful higher-order demons who have gained particularly impressive suites of powers and usually great wisdom and cunning to go along with it. The primary villains of the 'verse are demons who have evolved even beyond that - Nysrog is a god-like demon lord driven mad by a botched summoning who just wants to annihilate everything in his path, while Samal piggybacks on Nysrog's power but is more cunning, "serving" him only to advance his own desire for godhood (though Nysrog is more infamous among mortals, other demon lords, by and large, consider Samal the more dangerous of the pair). Higher-order demons are capable of procreating with humans; such hybrids sometimes, but not always, inherit their forebears powers and/or bloodlust. And if that's not enough, the demon realms are also sporadically at war with even worse things.
The Hollows: Demons are not mythological beings but a powerful race of Black Magic using shapeshifter aliens living in the novel's Alternate UniverseDark World the Ever After. They can be summoned to make a Deal with the Devil for things such as magical knowledge, assassination, or in one case giving legal testimony. Demons often attempt to manipulate the deal in order to take the summoner into the Ever After and enslave them as a Familiar which is considered a Fate Worse than Death. Later on, it is revealed that the demons are the ancestors of the Mage Species of the series. An ancient elven curse reduced the demons' fertility, resulting in their offspring being short-lived and unable to combine ley line magic with earth magic, cutting them off from the most powerful spells a demon can perform. These became the first generation of the witches.
Incarnations of Immortality: Demons can be created out of "ether" by the Incarnation of Evil and can look like pretty much anything he wants them to.
Inferno (Larry Niven and Jerry Pournelle): As in Dante, hideous humanoid demons, nine feet tall and with horns, hooves, tails, and tar-black skin, inhabit the lower reaches of Hell, guarding the damned and using pitchforks and long whips to encourage them along in their eternal punishments. A colossal demon, with teeth a foot long and a fingernail as long and sharp as a sword, is tasked with carving up the schismatics.
The Laundry Files uses "demon" as a catch-all term for some of the intelligences called down from the platonic realms. The books have so far featured unbound incubi (taking the form of an old lover), bound succubi (turn the host into a sex machine, but consume the souls of those they sleep with — and if not fed, they'll eat the host's soul), and lamia (reptilian kill machines). Most of the creatures are more akin to the Great Old Ones than conventional demons, though.
Malazan Book of the Fallen: The demons are any type of alien creature from Another Dimension. Aral Gamelon and the Shadow Realm are the realms most often mentioned as source for demons. There are many different kinds; the only commonality is that they can be summoned and controlled by sorcerers or alternatively bargained with. As a consequence, some of the demons seen in action are gentle souls who just want to go home to their old life as a humble extradimensional peasant, for example Lilac the Kenyll'rah in Midnight Tides. In Toll the Hounds, High Alchemist Baruk manages to capture a demon who looks suspiciously human and likely stems from the Crippled God's home realm.
The Mortal Instruments: Demons are very evil creatures. They come from their homeworld into the world of humans because they want to cause suffering and destruction there. They come in so many numbers and shapes that the shadowhunters they have been fighting for over a thousand years still do not know all of them.
When demons reach human's homeworld, they almost always look monstrous (unless they are shapeshifters). However, ordinary humans usually cannot recognize them for what they are. They can also summon demons, but it is rare that they actually enter into a pact with humans. Mostly they just kill them.
When demons reach people's homeworld, they almost always attack humans instantly to kill them, and sometimes even eat them up. However, the shadowhunters often notice it when demons are around, and hunt them. Sometimes the demons are also hunted and killed by werewolves.
Demons that are hardly more intelligent than animals prefer to attack humans immediately. Those who have a human intelligence seduce or rape humans so warlocks are born. Although warlocks are half demonic, they are as moral as humans. It is also believed that the fairies were originally created because demons mated with angels.
When a demon is injured, it bleeds black blood. And when a demon is killed, its body dissolves and returns to its home dimension.
All demons are superhuman strong, and often have a number of other powers as well. They are also immune to most common weapons. However, they are very sensitive to the weapons of angels, which is why shadowhunters can easily kill them with their weapons. In addition, sunlight also harms them, but not artificial light.
Some of the more powerful demons are fallen angels.
Myth Adventures: "Demon" is just a corrupted form of "dimension traveler", meaning that the term takes in a wide variety of species — and that humans who travel to another dimension are also regarded as "demons" there. Some demons look the part, like the horned, red-skinned, goat-legged Deveels, but even they're just shrewd merchants.
Nasuverse: Demons are rather unique in that they are described as an actual "substance" in and of themselves. Unlike other phantasm creatures (e.g. unicorns), they are created from the images of humanity and exist only to draw in disruptions. Possessions are actually the demons' attempts to draw pain and suffering away, but being "impossible" creatures, their manifestation generally causes harm and eventually kills the host. Then you have all the cultural variants which may also be labeled as demons but are completely different, like the ''oni'' in South-Asian countries.
The Nekropolis Archives: Demonkin are a type of Darkfolk. They come in a variety of forms; ranging from the classic Big Red Devil, to nightmarish amalgamations of insect and reptile parts, to truly abstract and alien conceptual embodiments. Most are capable of shapeshifting, but they are unable to disguise their distinctive kaleidoscopic eyes.
Night Huntress: There are two kinds of demons, those in their own corporeal bodies and those possessing hapless victims. A possessor cannot be killed, as they simply jump away into the nearest living thing when their current host dies, although they may be exorcised, or their host killed with nothing in range for them to possess. Corporeal demons are vulnerable to salt and cannot cross thresholds without an invitation, and can be killed if stabbed in the eyes with a knife made from demon bone.
Night Watch (Series): All demons are just Dark (i.e. selfish and egoistic, but not necessarily evil) magicians in their Twilight form. They can assume it at will when in "normal" world, and they automatically switch to it deep in Twilight. Since all magicians are born and grow up as humans, their appearance reflects the views of their society and Evil Makes You Monstrous is played straight. Thus teen delinquents in Dark Watch (or Face of Black Palmira) look like Big Red Devils from crappy movies.
Not all Dark Others become demons: witches and warlocks (the ones "connected with nature") always stay human, albeit the stronger they are, the older they look; Dark magician Edgar stays more or less human, which says a lot; Fafnir was a dragon; Anton describes one mook he killed as a humanoid lizard.
Then there are "lower Others" — generally not very strong in their narrow specialization, quite weak outside it and with a very limited choice over their form. One such creature mentioned in passing in Day Watch is a "beskud"note a creature from Central and Eastern European folklore — literally "miracle devil". Little info is given other than slitted pupils and triangular teeth. It is not elaborated if beskuds share lower Others' ability to convert humans to the likes of themselves. If vampires and werebeasts can do that, maybe so do beskuds.
Daevanote a kind of Central Asian demon or jinn is something else entirely — "a golem made out of pure energy". Very powerful, hard to control and near-indestructible, unless the magician knows a very specific spell. In medieval Middle East creating and controlling a daeva was a part of exam to the rank of Great Magician.
Devona or "daeva-possessed" has nothing in common with daeva aside from being Central Asian. This is a human infused with magic to be a better assistant to a powerful mage. The one we get to see is a master of combat magic and Obfuscating Stupidity. His introductory chapter ends with Heroic Sacrifice.
No Gods for Drowning: Glories are described as a sea demon of sorts and the lurking threat of the novel. They have appeared as grey-haired people with bronze skin, a huge red rash across their faces and huge wide eyes with red bumps. Glories hunt humanity and other sapient sea creatures because Glories enjoy hurting their prey and often mimic the voices of their victims to lure in others. Glories can also blind and mentally destroy the minds of people in order to increase their suffering before they go in for the kill.
Once: Nell Quick, self-employed nurse; natural remedy practitioner and psychic healer, seemingly to enhance her mystical influence, venerates traditional demonic figures. What we briefly see of one looks like a man - although looking it in the face, apparently, can be lethal.
Pact and Pale: Demons are frighteningly intelligent engines of complete destruction. Literally complete; whatever is destroyed by a demon is essentially erased from existence and cannot be recovered by any means—practitioners theorize that the observable effects of demonic action are actually the universe twisting itself to try to accommodate the sudden hole in reality. They also can't create, except to spawn motes which grow into more demons, though they can change and twist people and things. For these reasons, diabolists, who make deals with and use demons for power, are utter pariahs in magical society and rarely come to a good end.
Quantum Gravity: Demons live on another plane of existence, and Hell is not a physical place. Devils are best described as evil spirits, and are basically the opposite of everything that demons stand for. Demons make everything an art — you might not want to think too hard about that — and are very much for being one's self, regardless of what that is. Devils push people to make sacrifices (of self) now for some vague reward in the future, e.g. the greater good or prestige. A demon with (a) devil(s) cannot behave as a demon, and becomes an imp; imps push people through/around Hell. Demons come in many colors, and every shade means something different. Humans cannot become demons, but any creature who can interact aetherically can. Zal, for instance, is born elf but is also a demon.
The Reluctant King: Fourth level demons are invisible but capable of flight and great strength, fifth level demons like Rruakh resemble red-skinned winged humanoids with taloned feet and horns and no visible genitals (they sprout during the "right" season) who can't stand sunlight, seventh level demons are humanoids made entirely of solid fire and eighth level demons are vaguely described as massive, toad-like creatures. All of them are quite powerful but also surprisingly dumb, perhaps because, as Karadur speculates, their superior powers they were born with overshadowed any need to actually use their brains for anything.
The Riftwar Cycle: There are seven known levels of existence, with each being more dangerous than the one above it. Mortals come from the first level; demons are from the fourth and fifth (the overwhelming majority of demons featured in the series are from the latter). They are characterized by being incredibly vicious and living only for the id, and possess the power to absorb traits and knowledge from other living things they eat; there are several different "paths" a demon's evolution can take as it absorbs more powers, though they all seem to start out as imp-like beings, and will eventually reincarnate if killed. They are ruled in their own realms by Demon Kings, the most notable of whom are Maarg and Dahun, but can be summoned by sorcerers to the mortal realm (weaker demons are much easier to summon and compel- incredibly elaborate preparations are needed for an attempt to summon Dahun, and no attempt is even made to try and control him). A Kingdom Besieged had a young demon named Child as a major POV character though it turned out she is actually a dead human character reincarnated as a demon- exactly how it happened we don't know yet.
Rooks and Ruin trilogy by Melissa Caruso, in this setting over 1000 years ago 9 demons came into our reality. These demons are Energy Beings from a dimension primarily composed of energy and so when they entered the physical world, they went out of control and almost ushered in the end of the world. These 9 demons are known by their titles based on what ill they did to the world: Madness, Despair, Carnage, Nightmares, Disaster, Discord, Death, Corruption and Hunger.
Runemarks: Demons are so different they barely qualify for any of the traits listed above. From what has been mentioned, they originate in Chaos, are subordinate to Lord Surt (although Loki and Angie are pretty much just out for themselves), have Voluntary Shapeshifting, and are 'almost, but not quite the same thing' as gods, who are often referred to by the Folk as 'demons', mainly because the religion that has taken over from (what we would recognise as) Norse paganism in this universe would really rather people not make a distinction between the two.
Empire of the East and Book of Swords: Demons are powerful evil spirits, whose life essences can be stored outside them bodies, making them invulnerable unless their Soul Jar is properly destroyed. The demons used to be bomb explosions or other acts of destruction, the most powerful of them being a nuclear blast. When a general global nuclear war broke out, the United States activated a device called Ardneh that literally changed the laws of nature within the vicinity of planet earth to make nuclear reactions much less likely, preventing the chain reactions that make nuclear bombs, and nuclear reactors, possible. While this saved the human race, it also led to the collapse of technological civilization, and made magic possible and real. In this new, magical, world, the bombs became the demons. Fortunately, Ardneh still lived to fight them.
Sandman Slim has Hellions, which are the traditional fallen angels, kissi, who are the detritus of God's creation of the universe and feed on chaos and negative emotions and demons who are fragments of the Old Gods.
The Screwtape Letters is an exploration of what sort of society demons might have. The demons' physical form is not described (though when Screwtape gets particularly infuriated he becomes a large centipede, requiring him to fetch a secretary to take dictation), but Hell is sort of like a Police State full of Corrupt Corporate Executives — ordered under Satan, but still squabbling and infighting amongst themselves.
Second Apocalypse: Demons (called ciphrang) are incorporeal beings from the Outside who are summoned by Functional Magic to take physical form in the world. Ciphrang vary in power, but the strongest are a match for the strongest sorcerers. They find the physical world painful and hate every second of being inside of it. The cost of summoning them is that, after the summoner dies, the demon will torture him forever out of revenge. Some people don't think the cost that high in a world where sorcerers are considered damned anyway.
"Sefira" by John Langan features shapeshifting demons who are simply the natural, insect-like inhabitants of a place called the "Broken Plane." As it is beset by firestorms that last for millions of years, demons are eager to escape to our world via contracts with mortals, and must consume human souls to stay.
Shaman of the Undead: Demons are entities that live on the other side of the mirrors in various hells, although most seems to be ruled by the Tempter. They can possess people, and people "tainted" by black magic and demonic possession may become demons themselves. Most of them crave human flesh and souls, although some (like Bad Luck) satisfy themselves with pranks.
The Stormlight Archive: The Voidbringers. Shrouded in Myth, these things are generally agreed to be terrifyingly evil and powerful, but little is known about them. Words of Radiance implies that the Voidbringers are in fact Voidspren (small slivers of Odium) possessing various physical forms to affect the world.
The Summoner Trilogy: Demons are not always chaotic evil, but are instead Mons controlled by summoners after being captured from the Ether, their home dimension. They also come in numerous varieties that are actually many of the fantastic creatures of the setting, such as minotaurs, manticores, gryphons, and even drakes and dragons. There also the Ceteans or Old Ones, powerful and murderous Lovecraftian monsters who dwell in a deep abyss in the Ether. The protagonists, on their trip to the Ether, also find a temple with statues of creatures including mermaids, selkies, and angels, which are stated to be figures from different religions of the "mortal" world, but there is little information given, including to the characters, so it is unknown whether these were sapient demons, other denizens of the Ether, or merely myths represented by earlier travelers.
Tanith Lee has several races of demons in her Tales from the Flat Earth books. They live Underearth, ruled by the dark prince Azhrarn the Beautiful, Lord of Wickedness. The Vazdru, like their prince, seem like beautiful more-than-human beings, while their servants, the Eshva, live for their dreams and sensations; and the little Drin are master craftspeople somewhat like Dwarves. Her amazing tales of these not-always-malevolent beings seem similar to Babylonian, Egyptian, Persian and occasionally Russian and even Irish myth, but are in many respects like nothing that ever was.
Titan's Forest: "Demon" is used as a general term for the monsters and predators that roam the Understory and the Floor. In practice, these range from cunning and oversized but otherwise entirely normal animals to supernatural, primordial horrors such as chimeras.
Middle-earth demons are Ainur (angels) who joined Morgoth in rebellion against Eru Ilúvatar. Being shapeshifting spirits with various areas of influence, they come in several types, ranging from relatively weak vampires and werewolves to the immensely powerful Balrogs (and possiblyDragons too), and Sauron who seems to be (mercifully) unique in sheer power level. Saruman is also one of these... and totally vulnerable to getting his throat slit by a regular dagger.
The word "orc" derives from an old word for demon. Most of Tolkien's orcs were not demons of any kind (though it's implied that some of their commanders were orc-shaped demons) — rather, they were mortal creatures likely bred from Elvish and/or Mannish stock. However, Morgoth's Ring contained letters from Tolkien where he suggested that some of them were in fact demons, that is corrupted minor spirits given physical form, they were simply much less powerful than the ones that became Balrogs.
Melkor had corrupted many spirits — some great, as Sauron, or less so, as Balrogs. The least could have been primitive (and much more powerful and perilous) Orcs.
There are also rather odd beings of uncertain origin, like Ungoliant, a bizarre spider-monster whose nature is never explained. As all the evil in Arda was caused by Morgoth, she might even have come from somewhere else. Then there are the frightful, "nameless things" that "gnaw" the roots of the Earth, mentioned but never described by Gandalf.
The Tough Guide to Fantasyland: Their appearance is generally of a large size, with huge fangs, staring eyes, many limbs and an odd color. It's difficult to pin down however. They do appear to be physical, but of some gaseous substance that lets them squeeze through narrow spaces like keyholes yet not be harmed with ordinary weapons. Sometimes they are found running loose, though often must be summoned to our world then bound with Magic. Any sort of bargain with them is highly advised against, because all demons arecheats.
Quethnethogs, as they call themselves, are actually Wild creatures that look like large dromaeosaurids with opposable thumbs and emit a fear-inducing aura as a weapon - which is why they became known as demons.
Any creatures that possesses other creatures is known as a demon, even if it was human in life.
The Vagrant Trilogy: Unbound infernals are amorphous things with no clear personality. It takes effort for them to gain a form to protect themselves from the world. The most powerful are those who have found ways to survive; the Usurper possessed the corpse of Gamma and resurrected it (though it did a poor job), and the Uncivil wove herself a cloak of corpses that her cultists repair. In addition to the Usurperkin and the Uncivilkin, there are other lesser infernals with no allegiance and little intelligence.
Vainqueur The Dragon: They exist in the world, given titles like Chapter 27 being "The Demon King".
The Viper's Scheme features demons as a race with a separate culture and religion from humans, but the demons are not associated with a "devil" figure and are not any more evil than the average human (though it depends on who you ask). Demons are highly magical beings, but there's a lot of variation between the clans, from ones that look very like humans to those that resemble vampires or were-creatures.
In The Warded Man, the corelings rise up from the Core each night, and they, and their remains if they are killed, are destroyed by exposure to sunlight. They are almost immune to normal weapons, but can be kept at bay and even killed by wards, though the offensive wards were lost until Arlen rediscovered them.. They are also divided into various subtypes, most of which are associated with an element. They have animal level intelligence except for the mind demons, who are dangerously intelligent and Always Chaotic Evil.
When Demons Walk: The main villain is a demon, and for most of the plot it seems to be quite the average demon, but in the end, it turns out that all the demon's evil plans had the goal of returning home after hundreds of years of being bound to the human world.
Xanth: There are traditional "minor" demons (tempt humans, collect souls, horns, tails, etc.) and the Greater Demon X(A/N)^TH, a vast cosmic being who powers the entirety of Xanth's magic. The smaller demons are also not necessarily evil (though they usually lack souls). Males tend to be big bullies, although some like Beauregard can get along fine with mortals. Females tend to be more naughtily sexy than malevolent. And all bets are off should any of them receive all or part of a soul: it tends to make evil monsters into good guys. As for the big, cosmically powered ones, they are also not evil, although they basically view mortal creatures as insignificant ants, and use them as the pieces in their big universal chess game.
In The Chronicles of Amber series, "demons" is the term used to refer to all of the creatures that inhabit the Courts of Chaos, other than the Lords of Chaos themselves (a distinction which gets a bit blurry at times). Some look like the traditional horned and scaly monsters, but they run the gamut from talking cats to mathematical abstractions (they are creatures of Chaos, after all). The one thing they all have in common in that they are all capable of Voluntary Shapeshifting to one degree or another. This is not so much a magical power as it is an evolved survival mechanism for living in Chaos, where the environment is frequently subject to change without notice.
In Lord of Light, the demons are actually natives of the planet who lived there before humans came. They are disembodied creatures with the power to manipulate energy, and can possess people by controlling their neural activity, which they love to do so they can experience the pleasures of the flesh that they long ago abandoned. They are known as Rakasha, after the snake demons of Hindu Mythology, but when they choose to manifest, it is usually as a pillar of fire, not a snake. Their energy-manipulation abilities are similar to Sam's, but his are more powerful, which is how he earned the title "Binder of Demons".
Ro.Te.O has its devils possessing Elemental Powers and the standard bat wings. These guys fit definition 2 dot point 2 best although they aren't the creation of the Fallen Angels, rather they're the creations of this universe's Satan. Otherwise, they're pretty human... the main seven demons (and the two Fallen Angels in existence) going so far as to stand for each of the Seven Deadly Sins.
When the Angels Left the Old Country: At the beginning of the book, the narrator notes that sheydim (Jewish demons) are closer to what we might consider The Fair Folk. There are also Christian demons, and demons of any human culture that has them in its folklore.
Witch King: Demons are the denizens of Underearth, a mutable otherworld that can only be reached through Astral Projection. On Earth, they need to possess human bodies to interact with the world and can drain life with a touch, giving them a shady reputation in some areas even though they're not inherently evil or malicious.
(Jewish demons) are closer to what we might consider The Fair Folk. There are also Christian demons, and demons of any human culture that has them in its folklore.