Demons

Within the Nasuverse, the term "Demon" (魔) is used to categorize all creatures whose very existences cause distortions of the natural laws. Among this category there are those who existed in the world from the beginning, such as the Fiendish Kind, and those who were born later in response to the wishes of humans, the Demons. 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 (原初の悪魔でしょうか) have been referenced as the third strongest beings in the Nasuverse, while not including Servants, behind Arcueid Brunestud and the third personality of Ryougi Shiki. They have yet to appear in any works, and they have not been elaborated on in side materials.

Demons
Demons (悪魔) are the Sixth Imaginary Element. A force called forth and molded by the wishes of humans, the entities that are called as hunted down by the Holy Church. Rather than living creatures their existence goes more along the lines of a curse that acquired a physical form and psyche that is in accordance with the image created by humans in a fashion similar to the Elementals. Passive, imaginary existences that realize themselves by mimicking the concepts born from humans, at times they may even copy the human appearance. Still, the only function they are capable of is to cause distortions. Despite being called “demons”, the Devils are potentially beneficial to mankind as they absorb people’s pains.

Due the type the human images that they are based from, Devils are existences inherently flawed. If God is a perfect existence due the desires of people, then these demons stand on the opposite side of the same road: they are imperfect because that’s how people wished them to be. Thus it is a pointless exercise to destroy Devils, for they are born with a body that is already rotting away.

Probably also due their connection to human wishes, the Devils’ supernatural powers are similar in nature to the Wish-Granting Sorcery Trait of the Einzbern’s Holy Grail. But since they are originally just imaginary existences, it is impossible for them to harm a real, definite object by any means other than completely covering so to convert it into “nothing”.

Karyou Kaie
A Demon Master who is called "a genuine demon" himself. Similar to Merem Solomon, he has demon familiars that act as his limbs.

Four Great Demon Beasts
The familiars of the Dead Apostle Ancestor Merem Solomon.

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 always have a complete and healthy body, but a fragile mind.

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, there are those who are able to survive the process and eventually become capable of using the thing that is eating them, the Demon Masters.

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 distorting his soul the demon put his powers at the service of his host.

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 symptoms of "demon possession", which will only manifest on the breeding ground of the demon. 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 emotionall 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.

Fiendish Kind
Fiendish Kind (鬼種) is that which was a demon from the beginning, rather than something that was made a demon by humans. Something that is genealogically unrelated to humans and even the Phantasmal Species, but that are still considered natural creatures like any other animal. They declined as human civilization advanced and are now all but extinct.

Referred by the Holy Church as True Devils (真性悪魔) or Incarnated Demons (受肉した魔), they differ from the impostors that disguise themselves with human appearance by the fact that 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 call forth magic. As such, their powers are naturally superior to the Magecraft used by mortals.

Best known example is the Japanese ogres (鬼); in the past of Japan many families have crossbred with such creatures, resulting 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 Kishima Kouma, Tohno SHIKI, and Tohno Akiha. 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 Tohno Makihisa 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 Tohno Akiha's case, it also appears to simply happen without any obvious stimulus, as in Tohno SHIKI'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 Kishima Kouma. 

This term could theoretically be applied to conditions beyond Crimson Red Vermillion, including the fugue-like state Tohno Shiki 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.