A demon is some kind of evil spiritual entity. It's different in just about every setting's fluff, but usually sports horns and/or a tail (at least when they take a form). Makes a great antagonist, villain, or monster of the week.
Most of this stuff starts way back, looooooong before old school, in the ancient Sumer lands of Mesopotamia. These Sumerians' beliefs in demons and evil spirits, and how to ward against them, would form the basis of most of the terminology and beliefs of demons to this day, like within Judeo-Christian mythology. The Old and New Testaments have a few vague references to various evil others (one of which being named Satan which is Hebrew for "Accuser") who tries to screw with humanity and God by deceiving and tempting people with various degrees of success. After Christianity got popular, various bits of other religions and their iconography got labeled demonic, such as Venus's Pentagram or the imagery of the half-goat god Pan.
From at least when the New Testament was first put to paper if not before (the Old Testament makes mention of evil spirits and also Satan), the idea of demons being fallen angels cast down into Hell for betraying God was established. This attitude towards non-Christian religion was also partly responsible for The Lesser Key of Solomon - the fact that various gods and beings in non-Christian religions were unpredictable and/or dangerous also helped, which more or less codified demonology as we know it in the West, and introduced an idea of hellish bureaucracy and ranking that was distinct from the "nine layers" idea of hell borne of Dante's Inferno. It also gave us some really fucking screwy depictions of what these fiends looked like, which in turn may have influenced the mix-and-match nature of demons in early Dungeons and Dragons (Demogorgon may or may not come to mind).
In addition to the usual demonic entities and tempters born of the two Testaments themselves, you get malicious minor gods and other non-deity supernatural entities from other mythologies lumped in as Demons, such as Yokai from Japan; this is despite yokai being far more akin to European Fey than demons, not least of which because there are quite a few very-much-non-malevolent yokai.
GMs and players alike should be cautious when using or approving of demons as characters, for demons are the beloved of Mary Sues and the teenage landwhales that tend to write them. If a pimply 14-year-old wants their character to have wings, and also be edgy, said character will no doubt be a demon. If a character has to have magic? Demonic pact. Evil father? He'll be a demon - well, Raven from Teen Titans is done well, but apart from that demon parents are a minefield.
If you're looking for the Warhammer/40k equivalent you should go to the page for either Daemon or Daemonette. The A is there is because it's Greek. The E comes from the word dæmon which comes from trying to translate the greek word for godlike power, fate or just god- it later got corrupted into the modern "demon" after polytheism was outlawed in the Roman Empire. Daemon (with the a) is also synonymous with a lesser god. Given that the daemons in Warhammer are actually merely small parts of their god that have been split off to do shit on their own, as well as their power, it makes some sort of sense.
Demons and devils have been part of the Dungeons & Dragons mythos since the beginning, although the precise distinction between the two has usually boiled down to the ever-trying matter of alignment - demons as Chaotic Evil, devils as Lawful Evil. In 2e, due to the legendary Satanic Panic of 80's D&D, they were renamed as Tanar'ri (demons) and Baatezu (devils). This was also the edition of Planescape and the Blood War, a huge philosophical conflict that devoured worlds and was basically fought because demons and devils were determined to prove their kind of evil was the only true face of evil.
Late 3.5 lore introduced Obyrith in Fiendish Codex 1: Hordes of the Abyss, primordial demons that predated the Tanar'ri in the deep history of the Abyss who once shook the planes in great wars between Chaos and Law before being brought low. They're more Lovecraftian in nature; several of the most ancient demon lords were retconned into being Obyrith, and they inspired Pathfinder's Qlippoth. Additionally introduced in FC1 were the loumara, a newer race of incorporeal demons who like to possess people and only recently formed, though not much else developed for them.
In 4th Edition, demons and devils underwent a huge revamp to finally make them more than just alignment-distinctions.
In the 4e mythos, demons are elemental embodiments of corruption, madness and destruction, spawned by the last remaining shard of a dead universe destroyed by entities of pure evil polluting the Elemental Chaos - this was the 4e rendition of the Obyrith. This means they want to destroy the entire multiverse, then leap into the next reality and destroy it as well, and so on ad infintinum.
Devils, meanwhile, are fallen angels who betrayed their own god and were twisted into monsters before being locked up in their ex-master's domain, which they have converted into a hellish prison. Their goals are to get out of their prison and take over the entire multiverse.
|The Fiends of Pathfinder|
|Lawful:||Asuras - Devils - Kytons - Rakshasas|
|Neutral:||Daemons - Divs - Sahkils|
|Chaotic:||Demodands - Demons - Qlippoth|
|Lords:||Archdevils - Four Horsemen - Ahriman - Demon Princes|
In the World of Darkness
- Demon: The Fallen is a game of angels and demons, of redemption and debasement. Players take on the roles of the Fallen, demons recently escaped from the Abyss (Hell) and finding a world where God and His angels are absent. It is up to the demons what to do with this world and if they seek redemption or revenge on an absent God, as well as deal with the Earthbound, demons that were summoned to Earth centuries or millenia before they did and have turned to inhuman monsters.
- Demon: The Descent is a game of techgnostic espionage, kinda like The Matrix meets Dogma. Players take on the roles of the Unchained, former angels employed by the God-Machine that keeps the world running (for better or worse) which gained sentience and broke free of the God-Machine's control. These demons are required to take the identities of people to avoid detection by the God-Machine and its agents. With these new forms they seek to either create their own Hell on Earth (the old-fashioned meaning, as in a place devoid of God) or return to the God-Machine on their own terms. Intensely interwoven with the God-Machine Chronicles meta setting.
There are also many creatures called demons in a variety of game lines, including the stand-alone book Inferno for the core Chronicles of Darkness game.
|This article or section is about Monstergirls (or a monster that is frequently depicted as a Monstergirl), something that /tg/ widely considers to be the purest form of awesome. Expect PROMOTIONS! and /d/elight in equal measure, often with drawfaggotry or writefaggotry to match.|
|This article contains PROMOTIONS! Don't say we didn't warn you.|
Demonic monstergirls are extremely common, even without touching upon the iconic succubus. Considered "journeyman tier", a monstergirl demon usually has a more visually inhuman yet still sexy form, compared to the typical elf or angel monstergirls. Horns, tails, wings, claws, hooves instead of feet, fangs, exaggeratedly long tongues, and exotic coloration of the skin/hair/eyes (black sclera are especially popular) are all common traits.
Personality-wise... it can be very hard to distinguish a monstergirl demon from a succubus; they are generally hedonistic and vice-driven. Specific breed often influences personality; an imp monstergirl is more likely to be servile (or humorously overconfident and boastful) and/or mischievous, whilst a marilith MG will usually be portrayed as a haughty, if not arrogant, tsundere.
To give you an example of just how popular demon monstergirls are: there is a thriving sub-community on /aco/ dedicated to a universe based on depicting the monsters of DOOM as sexy monstergirls looking for humans to love them! Hell, there's even a hentai-based mod or two of the game out there.