- “Looks good, doesn't it? They're trapped in here. Terrified. Meat for the beast, and there's nothing they can do but wait… That's all they've been doing for days… waiting to be picked off. Having nightmares about monsters that can't be killed. But I don't believe in that. I always find a way. I'm the thing that monsters have nightmares about. And right now… you and me are gonna show 'em why.”
- ―Buffy Summers.[src]
A Demon, also called a Hostile Sub-Terrestrial, was a type of supernatural lifeform native to various Hell dimensions separated from Earth and generally considered to be evil due to their lack of souls and their destructive tendencies. The term was widely used to describe any creature that was not a deity, robot, unmodified human, or standard terrestrial animal. Though individual powers varied between different demon species, most, if not all, demon species possessed greater strength, endurance, and natural abilities than humans.
The original and most powerful demons originated eons ago. They were deitylike extradimensional beings called the Old Ones. The Old Ones lived on Earth during the Primordium Age after they were brought there from their home dimensions by the Seed of Wonder.
However, as time passed, new, mortal creatures began to appear on Earth and turn on and fight against the demons, and the Old Ones either retreated to other dimensions or interbred with humans, thus diminishing their powers through the generations. According to the Guardians, the last pure demon that walked the Earth during this period was killed by Sineya using the Mʔ.
By modern times, demonic creatures native to Earth were not full demons, but had a percentage of human heritage due to their ancestors' crossbreeding, while full demons, like Lower Beings, were native to other dimensions.
It's unknown what specifically constituted a demon. Maggie Walsh, the Head Scientist of the Initiative, refereed to demons as a “class” (“demon-class, Polgara species”). It’s unknown if by “class” she was referring to the specific rank in the taxonomic hierarchy. Anya Jenkins — an ancient demon herself — also claimed Sluggoth demons belonged to the same phylum as regular worms.
Demons came in a variety of shapes and forms with hundreds of known species of demons with diverse traits. Most demons were commonly bipedal or humanoid, very likely due to generations of interbreeding with humans; along with reptilian, mammalian, and insectoid features such as such as claws, rough skin, horns and sharp teeth. Most demons (obviously barring the Old Ones) were sized roughly similar to or slightly larger than humans, though could be as big as the Haxil Beast or even tiny like Gachnar.
Other demons shared an appearance closer to non-human animals: insectoids such as the She-Mantis, Queller demons, Ghora demons, Suvolte demons : arachnids like the Gavrok Box creatures, the Grimslaw demon, Ragna demon, Black Widower demon, and Najakot demon: reptiles like Machida, lizard demons, Kenny: and even imitated plant life like the Root demon. 
Like animals, demonic species could go extinct, such as the army of doom, Sluggoths demons, and Ragna demons. However, the last two were successfully recreated, using magic and selective breeding respectively.
Morality and behavior
By nature, the majority of demons were innately evil, merciless, and unsympathetic towards humans, animals and even some other demons. The reason for this seemed to stem from the lack of a soul which provided a sense of feeling culpable to one's wrong doings, evident by the fact that when humans became vampires they lost the ability to feel any type of general remorse. Confusing the issue was that most demons killed as a natural survival instinct, while others committed evil acts for their own recreational enjoyment, or religious/cultural reasons. Winifred Burkle even claimed that the act of a bug demon planting its offspring into the cranium of a Durslar beast was “all fine and Darwinian”.
For this demons were seen as purely evil creatures with death as the only case for dealing them. There were, however, some cases of morally sound demonic species or individuals such as Whistler, Lister demons, Jhiera, Kwaini demons, Val Trepkos, Mellish, Merl, Kamal, Codger demons, Yarbnie demons, Clem, Carlos, Kenny, Rory, Eldre Koh and Lorne (who appreciated human culture despite being a member of a violent tribe of demons). It was also shown that a soulless demon can reform so much they could form special bonds with humans, such as the vampire Spike who developed a strong love for Buffy Summers (albeit having it mixed up with a stalker-like obsession), leading him to fight for his soul back.
Like humans, some demons and even whole civilizations of demons could change their violent nature. Kamal was a violent demon who nonetheless became the protector of a woman who was fated to give birth to a savior. A species living in Quor'toth became pacifists by observing the father-son love between Holtz and Connor. The Anharrans demons were known for being blood-thirsty and archaic later became civilized and peaceful through Dawn Summers's influence. The Old One Illyria also gained faith in humanity shortly before she sacrificed herself.
Being transformed into a demon could also result in one developing their tendencies and a loss in their humanity. When Giles was transformed into a Fyarl demon, he shortly began to give in to the desire of killing people.
Some vengeance demons seemed to believe their acts of murder, torture and extreme unpleasantness were justified by the lesser crimes of humans. For example, Anyanka did evil deeds based on the wrong-doings of males, while Halfrek focused on the neglect of children. They themselves preferred the term "Justice demons". Though it should be noted that - in some cases - while Vengeance demons claimed to be acting on the behalf of the person they granted vengeance wishes for, the person who made the wish would sometimes end up falling victim to the wish itself. For example, Cordelia Chase made a wish that created an alternate reality where she ended up dying. Anyanka, who granted the wish, showed no remorse in that fact. Additionally, Halfrek was willing to leave Dawn Summers trapped inside her house after wishing that her friends and family wouldn't leave her, literally trapping them inside the house forever. It's likely that they weren't truly interested in making justice, but merely making excuses for the pain they caused. Interestingly, Anya showed disapproval over the Trio for robbing a bank, believing she deserved to get compensated appropriately for a hard-days work of causing pain and mayhem (though, this is more of an individual personality trait).
A demon's intelligence also varied among different species. A lot of demons such as Vampires, vengeance demons and various others possessed intelligence equal to humans while others like hellhounds (though, its Pylean counterpart appeared to be tamable), Drokken Beasts and Werewolves during their lupine period were purely, mindlessly feral and could be manipulated as such.
Demons reproduced in diverse and complex ways. Some species, including vampires, Wraith-er demons, and Werewolves, reproduced by infecting humans and then inhabiting them and using their bodies. Others, such as Haxil Beasts and Skilosh demons, infected humans in order to use their bodies to gestate their young. Some demon species reproduced via sexual intercourse. Some indeterminate number of these species, for instance Brachan and An-Movic demons, were cross-fertile with humans. Some demon species reproduced via asexual methods, such as budding. Some other appeared to have been created or manufactured by some other entity (magic practitioner, Old One, Power), and do not reproduce on their own. Thaumogenesis would result in the accidental creation of a demon from a spell, often one of a great deal of power where the said demon acted as a way to balance the effects of the spell.
Most demons were carnivores, mostly preferring human flesh as the best food source. According to Giles, some demons fed on the flesh of the dead to absorb their souls.
Some demons had diets tailored towards a specific substance, for example: vampires drank blood, Norman Pfister (in his maggot-form) devoured his victims completely within in a few minutes, Hellhounds feed on the brains of their victims, Lei-Ach demons were known to eat bone marrow, the Wig Lady enjoyed eating humans who ate the Doublemeat Medley, Sluks needed lots of water (particularly in human bodies), Gnarl ate human skin, Grimslaw demons fed on human hearts, Strom demons sucked the organs out of their victims, there was a finger eating demon, Tezcatcatl devoured the bloody heart of human heroes to obtain invincibility, Ragna demons captured and devoured their victims in a manner similar to actual spiders and various species were known to feed off human hearts. According to Willy, he was able to generate business by selling his demonic patrons fried chicken fingers.
Some demons feed on demon flesh. Lizard demons ate some of their young during hatching season, Drokken Beasts ate anything they preyed on, the Ano-Movic Clan had a time-honored tradition that involved eating the brains of their fiancee's previous husband (demon or not) in order strengthen their love by absorbing his, Sebassis drank the blood of his slave belonging to own his species and Black Widower demons engaged in sexual cannibalism.
Some demons practised cooking considered illegal in the human world. Examples include eating venues belonging to Richard Straley or Torg. When preparing for a negotiation between fueding demon clans, Wolfram & Hart employed a caterer specialized in ethno-demonic cooking.
Other demons had obscure and somewhat metaphysical needs for sustenance. Der Kindestod fed specifically on the life force of sick children, Gachnar fed on fear, Thesulac demons feed on the presence of insecurity, Shur-hod demons and the root monster fed on the life force of humans and even other demons, Clem fed on emotions, Soul Eaters were known for feeding on human souls, Polo and the Smile Time puppets fed on the life force of children, Lorophage demons fed on psychological traumas, the demon rock star Billy Rage fed on the fascination of his groupies, Hamelin demon fed on the family-related traumas of children, the Soul Glutton absorbed human souls and magical energy to strengthen himself, Najakot demon fed on magical energy, and Incubi fed specifically on the life force of human females.
Powers and abilities
The individual powers that demons possessed varied from species to species, but some appeared to be common to most, if not all species:
- Super strength — Most demons were noticeably physically stronger than humans, although the degree varied from species to species. (vampires, vengeance demons, Old Ones, Fyarl demon, Vyasa, Granok demons, Grappler demons, M'Fashniks etc.)
- Super durability — Because many demons were native to very harsh environments, their bodies tended to be more resistant to blunt trauma and injuries than human bodies, some species even resistant to bulletfire. Some demons had armored bodies, although they were relatively vulnerable if a part or all of the armor was damaged or destroyed. (Haxil beasts, dealer head demon, the Beast, Skip's species, Granok demons, Turok-Hans, Spike's crew, Space-bug-zompire thing)
- Super speed: — The ability to move super-humanly fast. (vampires, Soul Eaters, Doc)
- Longetivity/Immortality — A large amount of demon were shown to live longer than humans, and some of them were actually immortal, living for thousands of year without aging. (vampires, Vengeance demons, Mok'tagar demon, Boone)
- Empathy — The ability to sense or feel emotions. (some vampires, The Judge, vengeance demons, Empath demons, Deathwok Clan, Illyria)
- Telepathy — The ability to read minds. (Telepath demons, Ethros demons, Deathwok Clan, Sadecki demons)
- Teleportation — The ability to travel to another spot in space without having to transverse the distance between the two points. (Vengeance demons, D'Hoffryn, Mok'tagar demons, Granok demons, Stewart Burns, the Beast, Gnarols)
- Telekinesis — The ability to manipulate objects with the mind. (Eyghon, vengeance demons, Balthazar, Ethros demons, Sadecki demons)
- Pyrokinesis — The ability to manipulate and conjure fire with the mind. (The Judge, D'Hoffryn, Oden-Tal, Child of Words, Sweet, Local Boy)
- Cryokinesis — The ability to manipulate cold temperatures to create ice and snow. (Eddie Hope)
- Paralysis inducement — The ability to render beings immobile or incapacitated. (Ovu Mobani, Fyarl demons, Wig Lady, Gnarl)
- Possession — The ability to take control of another being's body. (Eyghon, Ovu Mabani, Haxil beasts, Ethros demons, Child of Words, Sluks)
- Precognition — The ability to see future events. (Deathwok Clan, Asphyx demons)
- Photokinesis — The ability to create and emit light. (Ovu Mabani, Shrouded Man, Cordelia, Pearl and Nash).
- Hypnosis — The ability to manipulate minds. (some [[vampires, Ovu Mobani, Thrall demons)
- Voice manipulation — The ability to manipulate once's or other's voices. (vengeance demons, Gachnar, Arney)
- Invisibility — The ability to make oneself unable to be seen. (Der Kindestod, Vocah, Thesulac demons, Pockla demons)
- Shapeshifting — The ability to alter one's appearance. (Marc, vengeance demons, Doyle, Ano-Movic demons, Deevak, Wraith-er demons, Eater demons, Lissa, Thricewise)
- Reanimation — The ability to raise the dead as zombies. (Ovu Mobani, Gachnar, Eyghon the Sleepwalker)
- Regeneration — The ability to quickly heal from injuries and regrow lost body parts. (vampires, Brotherhood of Seven, vengeance demons, Mok'tagar demons, Mohra demons, Deathwok Clan, Nahdrah demons, Jenoff, Strom demons)
- Levitation — The ability to rise above the air and stay in suspension. (Mok'tagar demons, The Gentlemen, Thesulac demons, Pockla demons, Brain Man, Cordelia)
- Flight — The ability to go airborne and propel oneself through the air. (Lohesh, Illyria's killers, Burge, Flying Monkies, Demonic Jetliners)
- Razor-sharp fingernails — The ability of having one's nails sharp enough to wound or shred flesh. (vampires, Der Kindestod, Vahrall demons, Gnarl, Turok-Han Vampires)
- Giles: “Oh… Anyanka… I-I beseech thee… to, um… In the name of all women scorned… Come before me… Oh!”
- Anyanka: “Do you have any idea what I do to a man who uses that spell to summon me?”
- — Giles calling forth Anyanka[src]
Certain demons could be beckoned, controlled and directed by specific methods. Tucker Wells trained a group of dangerous hellhounds to hunt prom goers by forcing them to watch prom-related movies. This influenced his younger brother Andrew who could summon demons to attack Buffy by using certain musical instruments such as panpipes and a didgeridoo. These demons include Jaarvlen Flesh Eaters (which he claimed was hard to control), Rwasundi demons, Glarghk Guhl Kashmas'nik demons  and an unnamed species of demon.
Angel command a Najakot demon for to defeat Amy Madison. Halfrek implied that Grimslaw demons were possible to house train, though difficult. Others demons, although having a free mind, were completely malleable by an higher being. It was the case of Glory's minions or the Zealots with Jasmine. Some individual demons could be summoned, but not controlled through certain spells. Anyanka could be called via a spell that required her necklace, while such worshiped figures such as Machida, Lurconis, "goddess" Yeska and Avilas could be brought upon for a ritual sacrifice and the Queller demons were only summoned to eliminate insane humans.
- Anya: “I made the rounds, myself. Tried to dig up anything useful from the demon community.”
- Xander: “They're a community now? What's next, a ladies' auxiliary? ”
- — Anya and Xander[src]
Most demons possessing near human intelligence adapted to most of the fundamentals of modern human society, but still retain traditions and attitudes that would be considered tribalistic and archaic. They had an underworld status and took part in human pastimes such as playing poker, though kittens were considered a delicacy, as well as currency, among demons. However, human money was still valued by mainstream demons(an exception being an unidentified M'Fashnik demon who, despite belonging to a mercenary race, considered money to be nothing more than paper). Also, pure innocence, especially that of a human child, carried a high deal of street value in hell dimensions. According to Anya, massacres were treated as social events by demons, similar to parties in the human world where demons would require invitations and even use these events to form romantic/sexual relations with other demons.
Most demons sported clothing of the modern era, though some, such as D'Hoffryn, Mok'tagar demons, Thesulac demons, Glory's minions, Pockla demons, Sahjhan, Rwasundi demons, the Fell Brethren and many more wore archaic robes. Feral demons (like Hellhounds, Polgara demons, Quellerjv, Drokken Beast and Bug demons) didn't wear any clothes.
During a social occasion, Anya wore entrails as if wearing a dress. She also claimed that burlap with blood larva was traditionally used as bridesmaids dresses in demonic wedding ceremonies where the grooms were forced to perform the rite of self-flagellation.
Interactions between other demons
Despite the degree of modernization, a lot of demons still relied on tribalism. They preferred to stay in packs or clans consisting of members of their own species that practiced ancient rituals and traditions that often involved barbaric sacrifices to their respective deities. They often waged feuds with other clans, as seen with the Deathwok Clan and Sebassis' species or the Viniji and Sahrvin clans, along with bigoted views of humans and demon species closely related to them such as vampires, who were less socially tolerated and even rejected by others demons.
For example, The Scourge considered them like the lowest of all the half-breeds, while the Ano-Movic Clan, Vyasa and the Hellions  were either offended or bothered to associate with them. Additionally, the Scooby gang found it odd that a vampire and a demon of a totally different species were fighting alongside each other. Rupert Giles himself claimed that, as rule, demons lacked empathy towards other demon species and even viewed vampires as abominations.
Even though they tended to abandon ancient demonic values and still enjoyed a naturally malevolent status, they did follow some loose rules when socializing with other demons as Spike was briefly rejected by his demon brethren for killing other demons after receiving his chip.
Integration into human society
Demons like Marc, Norman Pfister, Ken, vengeance demons, Kathy Newman, Talamour, Ano-Movic Clan, Doc, the Wig Lady, Wraith-er demons, Lissa, Helen Brucker, Kenny, Billy Rage or Incubus knew perfectly well how to disguise and to imitate the customs of human society.
Others demons were still motivated by outdated customs, such as The Tribunal Kamal who held to medieval codes of honor. Human sacrifice was something of strong religious significance to demons such as Carnyss Demons and Thrall demons. Other demons practiced human sacrifice even as a personal hobby or pastimes, such as in the case of Eli who dismembered virgins for fun during his free time.
The Ano-Movic Clan moderated their cult to live in the human society, tolerating even human beings in their family. However, they were still keen on continuing their culture's tradition of eating the brains of their bride-to-be's ex-husband (though only with his permission, which was given in the form of him proving his blessing to the new union). Certain demons were somewhat progressive, rejecting the ideas enforced by their species which they considered archaic and preferred to live among humans with whom they admired the freedom to make their own pursuits like Kathy Newman, Jhiera and the females of her species, Carlos, or Lorne. However, it's not always the case. Rieff complained that his only contact with the human world was during Halloween. In the case of Nash and Pearl, they considered themselves an evolved form of demons and felt superior to humanity and other demons alike.
Interactions with humans
Mainstream demons tended to mix with demons of other species as well as tolerate some humans who could accommodate their needs such as food from demon bars, treatment from Korean bathhouses and legal support from Wolfram & Hart. On the other hand, it was demons that serviced humans through their own special demonic talents such as Lorne's physic abilities, demon brothels which fueled certain men's fantasies and the demon Jenoff who granted humans their wish in exchange for their soul
Some demons offered power and wealth in exchange for live, human sacrifices. However, certain demons like Skyler had replaced these old values by cash money. The predominately demon biker gang, the Hellions, seemed to have possessed some human members, despite rejecting vampires due to their inability to remain in the sun.
Demons also seemed to have rather different views on beauty since the loose-skinned demon Clem found Buffy ugly because "her skin's so tight I don't know how you [Spike] look at her", Torg rejected Anya's offer of sex because he found her human appearance as unattractive despite once having a fling with her during her demon days and the inhabitants of Pylea found the half-demon Groosalugg's appearance to be monstrous and considered his human features as "defects" despite the Groosalugg being a physically attractive male by human standards, however, some demons have been shown to be attracted to or have relationships with humans. the demonic prostitute Lina who was willing to service the vampire Angel so long as he didn't change into his vampire face. The demonic princess Jhiera also demonstrated an attraction toward Angel
The Slayer posed a sense of fear among demons, especially vampires. Demons would use the Slayer as a Boogeyman figure to scare their children into behaving. When Buffy led a group of powerless Potential Slayers into a demon bar for training, she claimed her presence was the only thing preventing the demons from attacking them.
Ironically, some demons considered the human beings as monsters to them. When Halloween was celebrated in the premises of Wolfram & Hart, the demon Devlin dressed himself as a human being (using real human skin to that effect), aping them excessively to the point it irritated the human-loving Lorne.
- “Demons cling to old ways and ancient feuds, but are hopeless with technology.”
When it came to modern technology, the majority of demons remained inept with most advances that humans possessed. Though some demons, mainly vampires, made use of human inventions such as newspaper, television, and electricity. Though they made very little use of modern weaponry, usually limited to basic hand-to-hand combat and medieval weapons, such as swords and axes. Additionally, it was unusual for a demon to use bladed tools for disembowelment as they already had teeth and claws powerful enough to do the job.
On rare occasions, some demons made use of firearms. Examples of this behavior included Darla when fighting Buffy, Griff when collecting a debt, Lorne who killed Lindsey McDonald with a silenced pistol, M. James Menlo and Razor, the leader of a demon biker gang who used a starting pistol when destroying the Buffybot.
Human awareness of demons
For most of history, the majority of humans remained unaware or in denial of the existence of demons, with notable exceptions including the Watchers Council, the Initiative, the Slayers, the Scooby Gang, the Angel Investigations team, demon and vampire hunters like Sid, Charles Gunn, Daniel Holtz, Justine Cooper, or Robin Wood. Since the public revelation of demons, police departments included a paranormal force division.
Rupert Giles explained this as the tendency for people to "rationalize what they can and forget what they can't." Aimee Mann, a guest singer at The Bronze, remarked that she hated playing in "vampire towns." To those who did know, demonic culture and mythology was the subject of scholarly research with ancient books and texts documenting their history. Having had knowledge of his real nature, Harriet, the ex-wife of Doyle became an ethnodemonologist and had even become engaged to a demon. The website Demons, Demons, Demons also contained accurate information on demons.
The Sunnydale High School board and the police department also appeared to be aware of the supernatural goings-on in Sunnydale, as the police often covered up vampire and demon attacks under orders from Richard Wilkins. When Spike and members of the Order of Aurelius attacked Sunnydale High to kill Buffy Summers, Principal Snyder and the police chief wrote it off to the public as an attack by a gang on PCP, an excuse that has apparently been used before.
In Los Angeles, it seemed common that businessmen without scruples, gangsters or criminals, without counting the lawyers of Wolfram & Hart, were associated with demons in more or less doubtful business.
All the people present during the fall of Los Angeles in 2004 became aware of demons.
In the early 21st century, the existence of vampires, and demons by extension, became public knowledge and vampires instantly became the subject of a fad, capitalizing on the fact that many humans experience a thrill when bitten by vampires. Harmony Kendall became the face of vampires in the media, starring in her own reality show Harmony Bites. Since, all humans were aware of demons' existence on Earth. It was seen after the end of magic that demons were employed for various jobs like one, used by UC Sunnydale staff like debt collector to collect Buffy's student loan. In London, Parker and Tricia were racketed by an human associate with two demons.
Views of demons
Humanity's views of demons, like their views of one another, vary considerably depending on individuals.
Though the Scooby Gang and the Angel Investigations team were dedicated to destroying demons, they only sought to destroy those they saw as evil. Both teams employed the services of demons on several occasions and many key members of both groups had been demons. In fact, the demon members of Angel Investigations often outnumbered the humans. Both also argued that werewolves should not be held accountable for their actions in bestial form as long as they found ways of restraining themselves. Regardless, both sides were perfectly willing and able to kill demons for committing bad deeds; for example, when Anya granted a wish that resulted in the deaths of several fraternity boys, Buffy immediately decided that she had to kill Anya for it despite having peacefully talked Willow Rosenberg down after she did the same thing, justifying her actions by stating that Willow was human and Anya wasn't, and that as the Slayer, her decisions and judgement were absolute.
The Watchers Council refused to help Buffy Summers cure Angel of the Killer of the Dead poison on the grounds that he was a vampire, which resulted in her severing ties with the Council for over a year.
On multiple occasions, the U.S. government and its different groups showed a desire to study supernatural beings and find a way to use their power for military purposes, the most prominent examples being the Initiative and the Demon Research Initiative, its World War II-era predecessor.
Cordelia initially believed that all demons, besides Angel, were evil until she discovered Doyle's brachen demon heritage.
One of the strains on the relationship between Buffy and Riley Finn was that he automatically assumed that all supernatural creatures were evil, harshly criticizing the Scoobies when he discovered that they had been harboring Spike, though he realized that demons were capable of both good and evil just as humans were when he helped save a werewolf, Oz, from the Initiative, especially after he witnessed his superiors' inhumane treatment of Oz while he was in human form.
Anthony Harris' bigoted attitude toward Anya's demonic guests incited a riot between the two families. However, he was not aware of what they were really, considering them circus artists or a certain religious confession.