Demons

Within the Nasuverse, the term "Demon" (魔) exists within the law of nature and yet it is a power that isn't needed because it causes Disruptions in it's natural flow. It allows by being a part of nature, to warp (interference) the outer world by that connection to nature. Different species and types fall under this designation, but to those under the proper current, disruptions are perceived as evil. Len has already reached the point of being a disruption, but she was originally a familiar who acquired an aspect of a Succubus.

Among this category there are the True Demons (真性悪魔) who are born from nature, such as the Oni Kind, and those imaginary Demons born later in response to the desires of humans, from the Sixth Imaginary Element (第六架空要素). For the magi both of these types are, in practice, treated as one and the same.

Only the Church makes a more consistent distinction between them: the former are ignored while the latter are hunted under the precept of being existences that corrupt the work of God without being truly part of it. Horrors summoned by Prelati's Spellbook from an entirely different dimension with different natural laws are counted as demons. Beings called Primordial Demons (原初の悪魔) have been referenced as the third strongest beings in the Nasuverse, while not including Servants, behind Arcueid Brunestud and the third personality of Shiki Ryougi. They have yet to appear in any works, and they have not been elaborated on in side materials.

Demons
These Demons (悪魔) are passive, imaginary beings who are called forth and molded by the wishes of humans. Rather than living creatures, their existence goes more along the lines of a curse that acquired a materialized, biological impossible form whose ability and psyche, is in accordance with the images created by humans, in a fashion similar to a Heroic Spirits.

The natural ability of an imaginary demon is a Reality Marble (固有結界) and at the same time it is the alien common sense of demons (悪魔の持つ異界常識). But still, despite being demons they are potentially beneficial to mankind as they understand human suffering and take the people’s pains for themselves. This can be seen as a contract working under the terms of equivalent exchange: to compensate the demon for granting a wish, a human has to pay with his pain.

People who have bound a demon to themselves are known as Demon Masters. Known examples of Demon Masters are Kaie Karyou and the Dead Apostle Ancestor Merem Solomon who both have demon familiars that act as their limbs. But in order for these demons to become free, from their contract with a human, they need to be given a name by the human that bound them. Therefore you shouldn't give a demon a name that easily as it would be the same as admitting to the World that they are completely independent demons.

Shade
Shade (骸) is the form that the remains of Avenger take in the looped world of Fate/hollow ataraxia after he is defeated: a shadowy void that is barely shaped like a wolf-man.

Similar to demons, though much weaker in power. Still strong enough to slaughter any normal human, while someone with 10 years-worth of martial arts training might be able to last a few seconds in a one-on-one fight. The real problem is that it appears in massive numbers, having been born from a spiritual disruption meant to “create the same kind of monster”.

Demon Possession
Demon Possession (悪魔憑き) ,as opposed to the Japanese understanding of possession, in which spiritual beings such as a kitsune or inukami intentionally invade the body of a human to take control over it. This particular phenomenon works more in accordance with Western beliefs, in which something unnatural randomly attaches itself to something natural and corrupts it from the inside out. Humans cannot comprehend the reasons behind the possession phenomenon due to the randomness of how it happens and they might as well take it as an accidental event. The only known pattern is that the victims of possession are always people who have a complete and healthy body and are considered virtuous.

In its early stages, the only noticeable sign of a demon possession is a change in the host’s behavior caused by gradual loss of the capacity to reason as a human being. Being unreasonably offensive or coercive towards one’s surroundings are common symptoms at this point. If left alone, the demon inside the individual will start to imprint the shape of his own imaginary self over the body of his host. This process starts with subtle changes in the operations of the human body and culminates with mutation of entire parts of the flesh. The degree that these mutations can reach is determined by the demon’s strength. Because the demon’s shape is one that is biologically impossible, this transformation process usually results in the death of the host before the demon can fully mature. Still, in some rare cases there are those who are able to survive the process and become fully matured Demons and as their own souls are being consumed, they will cling onto something in order to survive.

Due their origin as imaginary beings, demons are naturally empathetic and sympathetic towards mankind. That is why they take away people’s pains for themselves and why the relationship between a possessed individual and his demon can be seen as a contract working under the terms of equivalent exchange: to compensate for devouring his soul the demon put his powers at the service of his host. The natural ability of an imaginary demon is a Reality Marble (固有結界) and at the same time it is the alien common sense of demons (悪魔の持つ異界常識).

The Holy Church sends out Exorcists to track down and deal with them. It is important for the Exorcists to find the demon before it can develop itself, for a fully matured Demon can not only corrupt the body of its host but also distort the mentality of those around, thus turning its living ground into a hell on Earth. However, only the host himself can be aware of the presence of the demon during the early stages of possession. And demons are cunning enough to suppress the spiritual disturbances that they would usually cause in order to prevent detection, so discovering a case of possession in its early stages is difficult. Spiritual disturbances like rap sounds in the surroundings or the sound of splashing water, much like the the presence of poltergeists, are more or less the results of a demon invading the body. That is to say, it's a disease that only affects the host. Much like a virus, it is a poison that does not spread to anyone else. A regular Exorcist lacks the necessary power to banish a fully matured Demon, so in these cases there is no choice but to call in an Executors to destroy both the host and the demon together.

Agonist Disorder
Agonist Disorder (アゴニスト異常症) is a mental illness in which the patient is unable to control his own emotions and is popularly referred to as “demon possession”. This illness only hits the emotionally weak people whose environment has weakened and it grows on their e xtreme negative emotions. In cases of mild severity, the patient show changes of personality due to loss of the ability to think coherently and arise of violent tendencies, which in turn leads to destructive behavior such as attempts of suicide. In more severe cases, the intensity of the mental disturbance starts to produce visible effects on the body.

In its initial stages, the symptoms of the disease resemble a lot true demon possession. The patient shows a boost or decrease of ego, possessiveness towards an individual, attrition towards his surroundings to the point that he can no longer be held down alone. Usually the disease remains as nothing but a mental disturbance, but in some cases the stress that these emotions cause distorts the functioning of the human body to start an overproduction of ligand (a type of neurotransmitter), which consequently becomes poisonous to the receptors. The body then tries to get around the problem by learning a new way to function that is unlike that of a human.

A patient that reaches such level of distortion might be able to attain a power beyond that of a human, but that is an irreversible path of self-destruction. Hypothetically speaking, if a patient regains an innate bodily function that was previously lost by action of the Agonist Disorder, the reset organ will have a shape and meaning far different from what it originally was even if the function remains the same. The activity of the new organ will place a burden on the normal parts of the body, leading to their breakdown and causing necrosis. For obvious reasons, not many of those who suffer from Agonist Disorder and reach this stage can survive without treatment.

It is possible to remedy the body of a victim of Agonist Disorder by removing the distorted part, except if said part is the heart. No treatments are viable if the heart of the patient is affected by the disease. However, because the very cause of the distortion is an over-activity of the body, the affected part is rather sensitive and removing it causes great pain to the patient. That same over-activity of the body also decreases the efficacy of neural anesthesia, making the treatment even harder.

The infection power of the pathogen responsible for the Agonist Disorder is the greatest among those known by man but, at the same time, its reproductive capacities are graded quite low in the CDC ranking. Because of that the number of victims of the Agonist Disorder has been quite steady since it was first discovered 20 years ago, neither increasing nor decreasing with time. The disease also seems to be geographically confined to the west region of Japan.

True Demons
These Demons are born from Gaia, with the law from nature, known as "Demon" (魔). It allows them by being a part of nature, to warp (interference) the outer world by that connection to nature. It is possible for creatures like Len, who was originally a familiar, to reach the point of being a disruption.

The Holy Church refers to them as True Demons (真性悪魔) or Incarnated Demons (受肉した魔), as they differ from the imaginary demons born from human desires. They are creations of God that were Demons even before being named so by people. Born to be Demons before being lifeforms, differently from humans they do not possess Magic Circuits-like attachments, but rather all their vital functions are mechanisms to perform the miracle of Magecraft merely by releasing prana. As such, their powers are naturally superior to the Magecraft used by mortals, the only exception is the Wish-Granting Sorcery Trait of the Einzbern's.

Demons are the greatest obstacle to mankind. If God is all-knowing and all-powerful, the demon is a being of human knowledge and powerlessness. It is the reachable but utterly incomprehensible abyss. In Fate/stay night, the shown demons are fakes, and the real deal, the incarnated demon is considered to be unmatchable by human beings. There's only one record of an incarnated demon being dispelled.

A demon is said to possess the human soul, and it will try to incarnate in the human body, but usually the mind of the human it is growing from is unable to take it and it ends up destroying itself while spreading its power in its surroundings.

Among these, there are people who from their own deeds cease being human and end up being categorized as demons, but this is called demonization so they are not True Demons. Whether it is possession or transformation because of their own deeds, there has yet to be one successful case of a phenomenon that has created a True Demon. In the true route of Fate/Extra CCC, Kiara Sessyoin underwent the transformation into a demon, but failed to reach the level of True Demons.

Though the terms "demon" and "demonic incarnation" are used, they are no different than higher dimensional beings from a different civilization on different planets. Their nature is closer to the evil gods of a certain fictional mythology popular in the 20th century.

Oni Kind
The Oni Kind (鬼種) are born from nature as demons and are counted as members of the Phantasmal Species. These creatures were symbolized by inferno and scorching heat. They declined as human civilization advanced and are now all but extinct.

In the past of Japan, many families wanted to crossbred with these creatures and a few of them made it possible. This resulted in the present clans of half-breeds, such as the Tohno and the Kishima. The relationship between the half-breeds and the oni is not unlike that between the Dead Apostles and the True Ancestors. Both the oni and the half-breeds have been in conflict with the Demon Hunter Organization for quite some time.

Inversion Impulse
The desire to enter an altered state of being, removed from conventional humanity, and the transformation into that state. This term is used specifically to refer to individuals with demonic ancestry, but may also be applied to individuals with other Transcendental lineages, or who have abilities related to altered states of consciousness. Known examples of individuals who have experienced an Inversion Impulse include Kouma Kishima, SHIKI Tohno, and Akiha Tohno. This term remains largely undefined: it is unclear whether this is a physical, mental, or spiritual phenomenon, if it is related to or alters the physical makeup or Magic Circuits of the subject, or whether it is permanent, though the actions of Makihisa Tohno in regards to his son suggest that it is a reversible occurrence. It is also unclear what causes an Inversion Impulse to occur. Though it can be brought on willingly, as in Akiha Tohno's case, it also appears to simply happen without any obvious stimulus, as in SHIKI Tohno's case.

Crimson Red Vermillion
Crimson Red Vermillion (紅赤朱), also known as Ancestry Return, is the state that appears when the demon's blood is maximized. They are enveloped by an illusion like fog, and the person won't be able to return to humanity. Having a thick demon blood, Akiha has an high chance of becoming one. When her hair turns red, she is about to become one, but she's barely controlling herself and keeping a balance between the human's blood and demon's blood. The only other known Crimson Red Vermillion is Kouma.

This term could theoretically be applied to conditions beyond Crimson Red Vermillion, including the fugue-like state Shiki Tohno experiences while under the effects of his Nanaya ancestry, or when a True Ancestor becomes a Demon Lord. The individual who experiences an Inversion Impulse exhibits the physical characteristics of their ancestry to a higher degree, gaining access to capabilities otherwise beyond their ability, at the expense of also suffering from the instinctual behavior associated with the source of their power.