Demons

''“Never trust a demon. He has a hundred motives for anything he does ... Ninety-nine of them, at least, are malevolent.”''

-Neil Gaiman, Preludes & Nocturnes

Born mostly from humans, demons are those who have sold their soul, usually to another demon... where and how the first demon came to be remains a mystery.

Physiology and Psychology
Called homomagia avaritiam, demons have been categorized as a human-based inhuman species and are considered "cousins" to other soulless/corrupted/undead species stemming from humans, such as vampires, rusalka, banshees, etc. Due to their soulless nature, they lack a natural conscience and must learn a moral code from those around them if one does not already exist from their human days, if they ever were human. This moral code can be easily twisted to suit their wants and needs, which are usually the most important thing to the inherently selfish demons. The majority of demons fall on the "neutral" spectrum, with the second most common alignment being various shades of evil.

As a human-based species in the homomagia genus, demons start their new life more or less resembling humans. Eye colors are odd, and are the first thing to change, into a random demonic color (from most to least common: black, indigo, violet, red, gold, white). However, demonic bodies naturally shift in response to the size of their hoard as their body undergoes natural defenses. Low level demons look human in order to lure deals in, and the larger and more powerful a demon hoard grows, the more the demon's body shifts from looking human to looking threatening and intimidating, in order to protect their hoard. Due to this, the age and general power level of a demon is easy to guess if one is aware of what to look for. Roughly in order, shifts in appearance that occur include: teeth multiplying, sharpening or growing; raised, tattoo-like lines all over their skin; iris and eye whites disappearing to create a monocolor eye; scales growing in on unprotected parts of the body such as the stomach, underarms, and breast; horns and spikes growing in; void-filled orifices (blackness when you look into a demon's mouth, ears, etc).

Diet
Demonic bodies view deals and trades as sustenance and while it isn't required, there is a feeling of need that causes many to believe that such deals are necessary. Most demons will have natural cravings for specific things (sexual energy, blood, information, body parts, money, etc.), though they can trade in anything. Those who try to ignore the feeling are more or less simply soulless humans. Feeding it only a little bit causes a happy, but weak demon, and those who indulge in making deals grow in power immensely.

Reproduction
Demons are one of the only species classified as "undead" who can naturally reproduce, and there are those who wish to and those who don't, as with humans. Two demons who have a child together will have a full demon child, and a demon who has a child with a human will have a half-demon (or "demonling") child. Demonlings can continue to have children with mortals or demons; if a demonling has a child with a demon, the child is considered full demon, but if a demonling has a child with a human, the child is generally a long-lived, strong, and magic-sensitive human. Neither demons nor demonlings can grow naturally, so demons who have children will often make deals to make humans younger in preparation for their child so that they can then gift their child enough years of aging to get them to maturity at least.

Subspecies
While "demon" is considered its own species, low-level demons who pass as humans often claim their own subspecies, usually based on why they became a demon, what sort of cravings they get, or both. This is purely colloquial and has no scientific basis that is so far known.
 * Fuccubi (gender neutral), succubi (feminine), and incubi (masculine) are low-level demons who make deals in order to gain energy. They are most often known for subtlety, working deals discreetly into sexual situations and exchanging sexual and romantic favors for this energy. However, not all fuccubi choose to take that route.
 * Empouse are demons who sold their soul for the love of an inhuman, oftentimes a vampire. Perhaps because of the vampiric connections, they often crave to add blood to their hoard.
 * Acheri are demons that find themselves drawn to sickness. They will often make deals to take it from people and later, to, thrust it upon others.
 * Devils, perhaps, are the most likely to become full blown powerful demons, and the rarest recognized subspecies. They often sold their soul for a pure purpose, and the bitter regret causes them to crave revenge against other pure humans. Any deal is alright, so long as it is twisted and thrusts someone into misery. Though the minority, they are what give demons the bad name they have.

Society
Like their "cousin" vampires, demons are territorial out of necessity, as too many demons in one area would not only cause all of them to go short on deals, but attract attention unnecessarily. Unlike vampires, this comes with a number of caveats... despite the territorial nature, demons are extremely social creatures, and can easily be driven to madness from isolation. Demons who desire to start families will do so happily, and will raise children as carefully as any human if at all possible. A demon who has no desire to raise a child will often still find a mate or platonic partner to spend time with, either by getting to know another demon, or coercing a human into selling their soul to become one. A family of demons as large as six or seven people might occupy a town; it is only outsiders that they have no patience with.

Family members will often answer to one matriarch or patriarch of their family, usually the person with the largest and best hoard. However, there is a streak of equality in that demon families all tend to respect each other equally, only deferring to the leader in case of conflict or a big decision. It has been shown, furthermore, that while demons tend to think of their family members as part of their own "hoard," they are, on average, highly loyal, and would almost never trade a family member away.