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The English term "Demon" typically denotes harmful or malevolent spiritual beings found in Judeo-Christian religious traditions. The term can also be used to describe the malevolent types of Jinn from Islamic traditions, or more broadly any harmful, malicious or frightening spiritual beings from non-Abrahamic traditions the world over. The modern term "Demon" is derived from the Greek "daimōn", which were spiritual beings in Greek and Roman mythology that were considered to be more powerful than mortals, but less powerful than the higher gods, such as the Olympians. In the original Greek usage of the term a "daimon," or "daemon" in Latin, described all such spirits, whether they were beneficial, harmful or neutral. As Christianity spread throughout Europe, Christians began translating Jewish and early Christian texts into Greek. In these texts multiple Hebrew words were translated into "daimon," with these words typically being related to evil or unclean spirits.
Many religions have a general idea and belief of demons, with some truths mixed into each of their stories. Demons were made by the God Valak before he ever made any of his other creations like vampires, werewolves, etc. Demons are malevolent spiritual entities that are corrupt, extremely evil, and very powerful. They are created from the accumulated misery and darkness born of the destruction of human souls, demons are universally malign and immoral in their actions and dealings. Born of torment and anguish, demons are creatures of malice and sadistic glee and have roamed earth for centuries while trapping humans in vicious cycles. Literally incapable of love or compassion, these beasts seduce and manipulate humans to fulfill what they believe is Valak's grand agenda. In truth, most demons only serve the will of a demon Emperor. There are a total of four Emperors; Jinzo, Abrax, Aziel, and Yubel, who are all known as the scariest, nastiest, and most powerful demons.
It is known that some demons have created offspring and in turn, created half demon human beings. Those individuals are known as Cursed.
Demonic possession is held by many belief systems to be the spirit possession of an individual by a malevolent demonic ability. Descriptions of demonic possessions often include erased memories or personalities, convulsions, “fits” and fainting as if one were dying. Demons possess humans to grow stronger. They feed off their victim's energy until the body burns itself out; and once that does happen the demon is able to steal the victim's soul for themselves. They also feed off the negative responses they elicit from others.
Human bodies controlled by demons tend to exhibit supernatural strength and abilities. Demons can keep their hosts alive even through injuries that would kill normal humans, and will only discard the flesh unwillingly or after fulfilling some goal. Being in human hosts, however, means demons are still vulnerable to things that can subdue humans like drug induced sedation. Demons who possess humans can be exorcised out of their hosts by an Exorcist. However, this only sends them back to where they came from and doesn't actually kill them, leaving the demon to learn and bide its time before it returns to another host.
Demons typically possess someone by appearing to them in their head as visions that only the person can see. They promise amazing gifts or things that the person would crave. They befriend the person as an imaginary friend until eventually gaining their trust and possessing them. While possessed a person is typically trapped by the demon in hellish visions, being tortured until they give over their soul. In some rare cases a person can hide within their mind, typically within a happy place or memory while the demon takes control of their body. The demon will fight to try and break into the safe space, and over time it will get in.
The possessed person slowly disintegrates and becomes a disgusting version of themselves. For a little while the person is still in control while the demon is in there with them, but slowly it begins to take control of their mind more and more. It causes them to do strange things such as hurt themselves, hurt others, vomit a dark green thick substance, soil themselves, among many other strange and horrible acts. Their body begins to rot from the inside out, their skin darkens with strange rashes and boils appearing all over them, their nails grow dark brown and black as their teeth also begin to appear blackened. A demon can cause someone’s eyes to become anything from having their pupils expand, their eyes go completely black, to orange, to yellow, to white, etc. It is not uncommon for strange slashes and bruises to cover a possessed person as well.
Supernatural Strength - A demon has enhanced strength when possessing an individual. However, as their host’s body begins to deteriorate their physical prowess weakens as well.
Levitation - A demon is able to hover off of the ground.
Telekinesis - A demon can throw objects and people across a room. The more powerful the demon is, the more they are able to move. They also use this to shake a room violently, mimicking an earthquake.
Telepathy - A demon is able to get into someone’s head, though this is not always easy. A person who guards their mind and uses faith to do so, can protect themselves from allowing the demon to read into their mind. However the demon typically only needs to get one piece of valuable information to use it against someone. They can create illusions once they do get into someone’s mind, that look, feel, and sound real.
Pyrokinesis - An uncommon ability, powerful demons are able to create and manipulate fire with their mind.
Electromagnetic Interference - A demon is able to manipulate electronics, causing them to go haywire. Examples being radio stations and television channels changing quickly, lights flickering, etc.
Storm Creation - An uncommon ability, powerful demons are able to create storms. The more powerful the demon, the more powerful the storms can be.
Biokinesis - An uncommon ability, powerful demons are able to manipulate a human’s body in order to cause them great pain by harming their inner organs.
Thermokinesis - Demons typically cause the temperature to drop around them severely, in some rare cases they may be capable of raising the temperature. Only the most powerful demons are able to manipulate the temperature to such degrees that things could catch on fire or cause ice to glaze over objects.
Insects - Demons have a strange connection with insects and typically summon flies, spiders, centipedes, and other things in the places they dwell.
Faith - Demons are weakened by a human’s true belief. That is why Exorcism works so well, as long as the Exorcist themselves believes in what they are doing. It’s their belief that gives items such as holy water, crucifixes, and prayer their power. If the person they are possessing doesn’t have faith it makes them easier to possess and harder to save.
The more people who believe in a method will make it more powerful. For instance, Catholic Exorcists are a very well known method to deal with demonic possession and because of it gives their methods more strength than a more obscure way. In other words, individual faith is important but overall faith of people in the world also matters.
Exorcism - A long process that can send a demon out of its host and back to where it came from. The stronger the demon, the harder it is to exorcise them.
Animals - Wildlife can sense a demonic entity, causing dogs to go crazy in a block radius or birds to fly into windows.