Demon

With the myriad of worlds connected to each other, it is no wonder that one of them contained creatures that are very similar to the mythological demons of humanity. The demons are parasitic beings preying on any living beings capable of feeling using the senses and feeling emotions.

Living parasites

Even in their own world, demons don't have their own material bodies. Instead, they resort to possessing objects as their intermediate hosts. Once they are picked up by a creature of higher functions, they can either pick it as their next intermediate host or as their definitive host. Once they pick a definitive host, they start to affect their mind and body, slowly taking over all of their functions. At that point, the only way to get rid of them is to go through an exorcism. But if a demon stays in one's body long enough, they might start to force their host to do anything they want, even before they take over their mind, by simply taking the functions of their organs as hostages. Physical changes will also start to show up on the host, reflecting the type of demon that possesses them, though the demons can hide those changes at will or even completely suppress their growth in the first place. Id demon's determinate host dies, they can move into one of their belongings, returning to the point of their lifecycle with an intermediate host until another suitable wearer shows up.
Demons tend to pick magical or valuable items as their intermediate host, increasing the chance of drawing the attention of the possible target. There also have been stories of demons related to gluttony that use food as their intermediate host. That way, they can spread as soon as they are consumed.
In their object form, demons are almost completely undetectable unless spells specialized in detecting them are used or unless they decide to use their powers, either to influence

Perception

Demon's ability to perceive depends completely on their host or lack of them. Once the demons get into the body of their definitive host, they are capable of feeling everything using their senses, even long before they take control over any piece of their body. While outside of any host, demons gain an ability unique to them that lets them perceive the value of items as well as detect any magical aura coming from them. And while they are inside an object, their perception depends completely on the structure of said object. All of them, no matter what item, can feel using the sense of touch and hear, though the quality of hearing depends mostly on the material the item is made out of. If an object has any kind of lens, reflecting surface, or painted eyes, the demon will be able to use them to see. The objects used for measuring of any kind let the demon perceive or feel the measured thing. Anything capable of creating sounds will let the demon mimic speech.

Dietary needs

There are many different subspecies of demons, but each one of them is related to one of the emotions or vices. Instead of consuming food, they have to feed on said emotion or vice. To do so, they either have to produce them inside of their host or cause them to appear in their surroundings. For example, the demons of wrath might do horrible things to other people to witness their wrath, and demons of gluttony might create hedonistic feasts.
Demons also have to remember to feed the body of their host even once they have taken complete control of it. Many young demons forget that and then they wonder why the body of their host dies. The dietary needs of the host stay the same as the ones before the possession.

Reproduction

Demons, even the ones related to lust, don't reproduce sexually. Instead, once they are inside their definitive host, they can start to excessively feed on their vice by indulging in it and making others participate in its expression. After they have their feel, they can continue the consumption getting close to their limit. When they feel they are about to burst, they can split into a bigger part and multiple smaller ones. The bigger part retains the memories and personality of the original, while the smaller ones have to come up with their own, becoming new demons. They will often haunt items belonging to the host of the original or will seek out some other intermediate hosts in the area.

Desire to dominate

Aside from some unique individuals, all demons possess the desire to dominate their host that is almost impossible for them to resist. As soon as the determinate host tales the possessed item, or worse, puts it on, the demons will want to dig their metaphorical claws into them and not let them go. If this desire isn't calmed down, the demon will always want to completely take control over the body of the host, and almost completely push them out. Just enough so that they still have to witness what the demon does with their body but at the same time unable to do anything to stop them in any way.

Harmless coexistence

Since the end of Great Fae War, more and more worlds require treating their inhabitants morally. As such there have been many restrictions placed on possession, and anyone who oversteps them will be hunted down and brought to justice. Demons have to adapt to the world that changes around them. There are quite a few ways for them to get a host.

Hostless living

The simplest and most legal way is to get either an artificial body or a permit to haunt a corpse. At first, the artificial bodies couldn't work as anything more than intermediate hosts as they didn't have any emotions. But as the years pass and magic and technology develop as the new species get introduced with portals to new worlds.l It is now possible to create a golem capable of feeling emotions but without any core that would give them life on their own. And so, a demon can possess such a golem without overwriting anything they are.
The demons who would prefer to have more meat on their bodies can apply for a permit to haunt a corpse. The process is long and complicated and the line for potential recipients is even longer, especially since there is also an interview with the family of the corpse. They have the final say if they want the specific immaterial person to possess the body of the one they lost. But if they agree, the corpse is sent for preparation to make it "fit" its new owner as neatly as possible.

Creating a contract

If a demon still would prefer a host, they can create a contract with a possible host. Contracts like that usually go over the permissions to perform various acts, both legal and illegal, while the demon is in control of the body, the permitted allocated time given to the demon, and permitted changes to the body. Contracts like these are mostly valuable only to the ones who created them, but can be used in court should one of the sides not fulfill their obligations.

Interviews and speed dates

But how does even a single demon pick up a potential host? As the number of immaterial creatures searching for hosts began to grow, both governments and various organizations started creating programs that would help them find one, ensuring both the highest compatibility and safety of the host. Some of the programs go through a temporary possession and various tests just to make sure everything goes right as well as trainings that let the demons overcome or . And so, a demon can sign up at the office that helps immaterial creatures. Once they do, they will start getting invites to interviews with potential hosts or even a new kind of "speed dates" for those who always wanted to check their chances of being with an immaterial creature but never had a chance before.

Being a passenger

Instead of taking over the body or having control over it dependent on a contract, the demons of some kinds might choose to only be passengers of their hosts only asking for some more control whenever something related to their vice is being done. And so, a demon of gluttony might only take partial control during eating and sloth demon might want to awake from its slumber whenever it is time to relax. Those demons are quite rare, as they are ones capable of ignoring their desire to dominate either through their will, their lack of any significant power, or by being born that way.

Origin

Demonic worlds

Spread

Right now, they can be encountered in most of the worlds where sapient beings are present. They tend to pick the worlds that are crossroads, tourist attractions, trading worlds, or are in other ways either heavily populated or experiencing a lot of movement of beings of all possible species. Though they avoid any Fae worlds, having bad experiences with them during and before Great Fae War.

Appearance

While demons might have preferences for the bodies they inhabit, either picking the ones that suit them the most or modifying the ones they are in, by themselves, they do not have any shape or appearance. There are spells and devices that let one see them whenever they don't have a host. Those things show them as constantly shifting clouds that sometimes take on the appearance of a horned humanoid. And once in a while when they are in their intermediate hosts (objects) the aura of stronger demons can be physically visible, showing up on the object and around it as flames or signs related to the said demon. Similar spells and devices as before are needed to see the aura of the weaker ones.

Examples of changes in the host's body common among all the demons

• Horn growth or additional pair growing out
• Skin changing color and its texture becoming smoother
• Growth of tail or the tail that already exists becomes narrower and grows a tip (the shape of the tip changes from one individual to another)
• Very rarely, a growth of wings or an additional pair of them
• Markings showing up on the body
• Eyes changing their colors and the shape of iris

Examples of changes in the host's body dependent on various demon types

• Gluttony - Changes related mostly to mouth, like additional teeth growing out, teeth changing to those of omnivores (though the dietary needs of the body don't change), changes in muscles of the jaw that will let the body of the host unhinge their jaw, changes to stomach that increase its resistance to any food-related sickness, as well as make the digestive tract more efficient.
• Wrath or violence- Increase in muscle mass, mostly focusing on forming muscles used in combat. Possible bone-like protrusions and hard exoskeleton-like spots on fists, feet, legs, and arms, both for attacking and protecting oneself
• Envy - Increased number of eyes, making the body more malleable and able to change when reshaped like clay, growth of tentacles
• Sloth - Constant bags under eyes, body turning softer and more resistant to falls caused by falling asleep, possibly longer limbs, so the demon doesn't have to move too much to reach things and has to make fewer steps when walking somewhere
• Lust - Changes related to increasing attractiveness of the host to reflect the features that are considered most attractive among their species
• Greed - Metallic (golden, silver, bronze) sheen of skin, fingernails, and hair, with possible single strands of hair turning the same color, elongated fingers

Demons and Fae

As most species that had to deal with them before, demons steer clear from any places where Fae might be present. Back before the Great Fae War ended, it was popular for Fae to lock the demons in their intermediate hosts, forcing them to be living jewelry, ornaments, furniture, or magic items. Additionally, since demons bind themselves to their hosts to the point where their nature is also affected by the changes to their host, the magic of Fae was especially strong against them. They could change and break them against their will, destroying anything that made them them, leaving empty shells that could barely still be called living.