Demons (悪魔, akuma?) refers to entities founded within the 6th Imaginary Aspect (第六架空要素, Dai-Roku Kakuu Youso?). Not to be confused with Aberrations (魔, Ma?), a more general category of existences.
Demons (悪魔, akuma?) are existences founded within the 6th Imaginary Aspect (第六架空要素, Dai-Roku Kakuu Youso?), called forth by human desires and wishes, that materialize as Manifest Phantasms (実像幻想, jitsuzou gensou?) formed from the thoughts of humanity. In general, they act to realize the wishes they come to be invested within by distorted means.
Unlike True Demons, they are existences that obtain being by the imposition of a Unit Designation (個体名, kotai-mei?). As Demons attain true freedom upon receiving a Designation, it is a general rule that the naming of Demons is not an act to lightly engage within. For example, in Fate/Grand Order, Goetia describes his component Demon Gods as existences that had obtained discrete Names and Powers.
Temporarily setting aside the outcomes of engaging with such, Demons may be assessed as allies of humanity, who exist so as to empathize with and resolve the sufferings of humanity. Despite this, the concept of Demons (悪魔の概念, akuma no gainen?) is by humanity treated as the greatest of obstacles.
So as God is to omniscience and omnipotence (全知全能, zenchi zenou?), so as demons as an existence are to mortal levels of awareness and impotence (人知無能, Jinchi Munou?, lit. "human intellect and impotence"). They are to humans as an abyss (淵, fuchi?) that lies within arm's reach, yet decisively beyond comprehension. The manner of existence taken by "Demons" and "Fiends" is in essence indistinguishable from that of a higher-order lifeform (高次元生命体, kou-jigen seimeitai?, lit. "higher-dimension lifeform") that inhabits a different sphere of civilization (異なる文明圏, koto-naru bunmei-ken?) upon a different planet (異なる惑星, koto-naru wakusei?).
In sum, said existences can, per the recognition of the Universe of Awareness (認識宇宙, ninshiki-uchuu?, lit. "the universe derived of observation / perception"), no longer be discerned as "human." Their existences would approximate to that of the horrors (邪神, jashin?, lit. "evil god") of a certain fictitious Mythos popular in the 20th century.
In general, demons are existences that take possession, utilizing the corpus of a human to incarnate (受肉, juniku) and take action, but in the event that the mind of the host cannot withstand the strain, it is common for the self-destruction of the host to occur, dispersing the essence of the demon into the vicinity. There exist also humans who by way of their own actions are placed unto the category of Demons as a result of losing their humanity.
Upon possession by a demon, irregularities manifest in the the victim's body, eventually rendering them as grotesque monsters by the metamorphosis of the flesh. High-ranked demons, however, are capable of suppressing the manifestation of these symptoms, making their detection difficult, and it is likely that a disaster becomes inevitable in the event of their discovery.
In the world of Fate/Extra -- whether by way of Demonic possession, or in the circumstance that one's own actions bring about a transformation into a fiend -- it is said that the process whereby a True Demon is birthed unto the world has, as of present day in 2030, not been brought fruition even once.
Conversely, the wording in Extra CCC Material suggests that if the psyche of a human host is capable of withstanding possession without collapse, it is possible for a common Demon to be uplifted as a True Demon (真性悪魔, Shinsei Akuma?).
True Demons (真性悪魔, Shinsei Akuma?) are demonic beings that can exist without obtaining a Unit Designation (個体名, kotai-mei?) from humanity,, or cladding themselves in the Ideations of Man (人間の想念, ningen no sounen?).
Distinct from those Demons that appear Stay Night, which exist only as imitations, these are the real thing -- Incarnated Demons (受肉した悪魔, juniku-shita akuma?) with which humans can't hope to compete with. In Fate/hollow ataraxia, they are said to be authentic Demons brought forth by the Lord, who were as they were before being named by Man.
Unlike humans -- who exercise Magecraft by way of Thaumaturgical Circuits that are essentially pseudo-nerves fabricated after the body's nerves -- True Demons possess nothing so much as Circuits. The entirety of their vital processes serve as faculties that exist for the purpose of calling forth aberrations, and they exercise a Magecraft of an order beyond human ability.
Demon Possession (悪魔憑き, Akuma Tsuki?), 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.
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 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 (アゴニスト異常症, Agonisuto Ijousho?) 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 extreme 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 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.
- ↑ 1.0 1.1 1.2
 JP Wiki: Demon
- ↑ 2.0 2.1 2.2 2.3 2.4
 Jonnobi: Demon
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8
 Fate/EXTRA material - Encyclopedia: True Demon [Circumstances], p.190
 Fate/Grand Order - Final Singularity, Chapter 13
- ↑ 5.0 5.1 5.2
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