Daemons 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 Daemons (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 daemons 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. Yet they are not to be confused with Demons (魔, Ma?), a more general category of existences.
But the manner of existence taken by "Daemons" and "Demons" 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 the world of Fate/Extra -- whether by way of Daemonic possession, or in the circumstance that one's own actions bring about a transformation into a Demon -- it is said that the process whereby a True Daemon is birthed unto the world has, as of the present day in 2030, not been brought to 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 Daemon to be uplifted as a True Daemon (真性悪魔, Shinsei Akuma?).
Daemons (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 get born from their wishes within their human psyche. They act to realize the wishes they come to be invested within, because of their alien common sense, by distorted means. They gain a form fabricated by human thoughts and then materialize as Manifest Phantasms (実像幻想, jitsuzou gensou?). In general, a daemon usually takes possession of the human that birthed it, utilizing it's corpus to incarnate (受肉, juniku) and to 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 daemon into the vicinity. Upon possession by a daemon, irregularities manifest in the the victim's body, eventually rendering them as grotesque monsters by the metamorphosis of the flesh. High-ranked daemons, those who have a Unit Designation however, are capable of suppressing the manifestation of these physical symptoms, making their detection difficult, and it is likely that a disaster becomes inevitable in the event of their discovery.
Then unlike True Daemons, they are existences that obtain being by the imposition of a Unit Designation (個体名, kotai-mei?). As Daemons attain true freedom upon receiving a Designation, it is a general rule that the naming of Daemons 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..
Yet there are a few ways for employing daemons as familiars. One of them is the Magecraft known as Spiritual Evocation (降霊, Kōrei?) taught by the Clock Tower, the headquarter of the Mage's Association, based on the knowledge of what King Solomon left behind. While in Tsukihime, exist the Four Great Daemon Beasts of the Dead Apostle Ancestor Merem Solomon who he imagined and brought into reality through his Demonition (デモニッション?) ability, they continue to serve him out of debt and loyalty without a Designation. In DDD, there exist the Four Daemon Limbs who are bound to Kaie Karyou because his wish, of gaining a 'human form', was realized by turning his emotions into daemons and each daemon received a Designation accordingly with the emotion that they were made from.
True Daemons (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 Daemons that appear Stay Night, which exist only as imitations, these are the real thing -- Incarnated Daemons (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 Daemons 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 Daemons possess nothing so much as Circuits. The entirety of their vital processes serve as faculties that exist for the purpose of calling forth demons, and they exercise a Magecraft of an order beyond human ability.
Daemon 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 unintentionally 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. However it is mostly caused by a Daemon, who takes possession of a virtuous human with a healthy body.
In its early stages, the only noticeable sign of a daemon 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 daemon 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 unthinkable 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 daemon's strength. Because the daemon's shape is one that is biologically impossible, this transformation process usually turns them into grotesque monsters and the host dies before the daemon can fully mature. Still, just like how magi can become vampires, there are those rare cases who are able to survive the process and become fully matured Daemons. They continue to survive even as their own souls are being consumed.
Daemon Possession can not only corrupt the body of its host but it also causes many spiritual disturbances like rap sounds in the surroundings or the sound of splashing water, much like the presence of poltergeists. These are more or less the symptoms of a daemon possession rather than daemon possession itself. Daemon Possession is a disease that only affects the host. Much like a virus, it is a poison that does not spread to anyone else. While the spiritual disturbances caused by a daemon, can 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 daemon during the early stages of possession, for it isn't known by others that someone is possessed until they transform. And daemons are cunning enough to suppress the spiritual disturbances as much as possible, that they would usually cause, in order to prevent detection until they reach maturity. So discovering a case of possession in its early stages is difficult.
The Holy Church sends out Exorcists to track down and deal with them. It is important for the Exorcists to find the daemon before it can develop itself. For this reason values the Church people like Caren Hortensia who can act as a daemon detector and employ them as assistants for the Exorcists. For a regular Exorcist lacks the necessary power to banish a fully matured daemon, so in these cases there is no choice but to call in an Executors to destroy both the host and the daemon together. The only known Exorcist to ever exorcise a full grown daemon from a person is Mr. Dawn from the Burial Agency.
- ↑ 1.0 1.1 1.2 1.3 1.4
 Jonnobi: Demon
The 6th Imaginary Aspect. Existences born of the wishes of humans, passively (受動的な, shuudou-teki-na?, lit. "as acted upon") called forth by human desire. Manifest phantasms (実像幻想, jitsuzou gensou?) that take form via the aggregation of ideation (想念, sounen?, lit. "thoughts"). For materialization (実体化, kittaika?, lit. "manifestation to material") to occur, "a form fabricated by humans" is necessary. Not True Demons, but existences that obtain being by the imposition of a Designation (個体名, kotai-mei?, lit. "unit designation").
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.
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.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7
 Fate/EXTRA material - Encyclopedia: True Demon [Circumstances], p.190
True Demon [Circumstances]
Even in the world of Extra, 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 as an abyss (淵, fuchi?) that lies within arm's reach, yet decisively beyond comprehension.
Distinct from those demons from Stay Night, which exist only as imitations, the real thing -- Enfleshed Demons (受肉した悪魔, juniku-shita akuma?, lit. "Incarnated Demon") are that with which humans are incapable of competing 太刀打ちできない (tachi-uchi-dekinai, lit. "incapable of crossing swords"?).
(※ There is in record only a single instance in which an Enfleshed Demon was forced to disperse (退散, taisan?, lit. "disperse / retreat"), but that's a different story.)
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 psyche (精神, seishin?) of the human host (苗床になる人間, naedoko ni naru ningen?, lit. "the human that becomes a seed-bed") cannot withstand the strain, it is commonplace for self-annihilation (自壊, jikai?) of the host to occur, dispersing the essence of the demon into the vicinity.
There exist also humans who by way of their own acts (業, gō?, lit. "sins; worldly preoccupations / fixations / absorptions; karma") are subsumed unto the category of Demons as a result of losing their humanity; but these creatures that are said to have become Fiends (魔人, Majin?) cannot be considered True Demons.
In the world of Extra -- whether by way of Demonic possession, or in the circumstance that one's own actions bring about a transformation -- the phenomenon whereby a True Demon is birthed unto the world has, as of present day in 2030, not been brought successful fruition even once.
Though the classifications of "Demons" and "Fiends" do exist, their manner of existence 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 cosmic horrors (邪神, jashin?, lit. "evil god") of a certain fictitious Mythos popular in the 20th century.
In the True Route of CCC, a certain individual that has undergone transmigration did indeed manage to reach this realm, but owing to a slight mistake, they were capsized from the Seat of Divinity (神の座, kami no za?).
- ↑ 3.0 3.1
 Colorful Moon Tsukihime: Ma
- ↑ 4.0 4.1
 Melty Blood Act Cadenza PS2 Manual - Dictionary: Reality Marble [Unusual talent], p.056
Reality Marble [Unusual talent]
Intrinsic Field. Originally referred to the alien common sense possessed by the beings called devils, but now also includes the unique boundary fields possessed by many people.
A subset of marble phantasm, it erodes reality and gives form to the user's imagined world. It is the sorcery closest to magic, and a forbidden art within the Association.
 Fate/Grand Order - Final Singularity, Chapter 13
I was omnipotent.
It was not omnipotence possessed because Solomon created me thus.
We were originally born of Distortions within the activities of the human psyche --
a Collective of Demon Gods (魔神, Majin, lit. "jinn") which had obtained Names and Powers.
I was from the beginning a Transcendent One,
and when We were become, omnipotence and omniscience were had.
名称と能力を獲得した 魔神（もの）を束ねた 群体（もの）。
- ↑ 7.0 7.1 7.2
 Fate / Hollow Ataraxia
That which is called Prana is merely a fuel for the purpose of activating Magecraft; by itself, it can wield no effect.
However, there do exist exceptions to this rule, whereby -- restricted to those circumstances in which Prana itself comes to bear qualities that approach the nature of Magecraft -- it is let to take a 'form' (カタチ, katachi?, lit. "shape").
For example ――― it is rumored that the Einzberns bestowed a personality unto the Holy Grail.
If said personality is possessed of Prana, it would from its birth come to bear the Thaumaturgical Trait (魔術特性, Majutsu Tokusei?) of "Wishgranting" (願いを叶える, negai wo kanaeru?).
Should its vital processes and the faculties that it holds as a Holy Grail come into direct junction, it would likely be able to give 'form' to the Miracle (奇跡, kiseki?) of "Magecraft" merely by the release of Prana.
The "Incarnated Demons" (受肉した魔, juniku-shita ma?) spoken of by the Holy Church also follow this convention.
Those beings created as fiends before existing as organisms exercise a Magecraft of an order beyond human ability.
The Thaumaturgical Circuits that Magi (we) bear are little more than spurious nerves (疑似神経, giji shinkei?, lit. "pseudo- / quasi- / virtual nerves") -- fabricated (作り上げた, tsukuri-ageta?) in the shadow 裏 (ura, lit. "shadow / reverse / behind"?) of the nerves that we possess as of existing as humans; a faculty extrinsically affixed (後付, atozuke?, lit. "attached after").
However ――― fiends possess nothing so much as 'spurious nerves.'
To them, the entirety of their vital processes serve as faculties that exist for the purpose of calling forth fiends.
It appears that the Holy Church refer to such beings as "True Daemons" (真性悪魔, shinsei akuma?).
Distinct from those imposters (偽物, nisemono?) that clad themselves in the Ideations of Man (人間の想念, ningen no sounen?) and come into being (成る, naru?) via "Unit Designation" (個体名, kotai-mei?, lit. "unit name"), they are authentic "Daemons" brought forth by the Lord, who were as they were before being Named (名付ける, nazukeru?) by Man.
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