Within the Nasuverse, the term "Demon" (魔, Ma?) exists within the law of nature and yet it is a power that is not needed because it causes Disruptions in its 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 Devils (真性悪魔, Shinsei Akuma?) who are born from nature, such as the Oni Kind, and those imaginary devils born later in response to the desires of humans, from the Sixth Imaginary Element (第六架空要素, Dai Roku Kakū Yōso?). 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, Gensho no Akuma?) 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.
These Devils (悪魔, Akuma?) 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 devil is a Reality Marble (固有結界, koyuu kekkai?, innate bounded field) and at the same time it is the alien common sense of devils (悪魔の持つ異界常識, akuma no motsu ikai joushiki?). But still, despite being devils 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 devil for granting a wish, a human has to pay with his pain.
People who have bound a devil to themselves are known as Devil Masters. Known examples of Devil Masters are Kaie Karyou and the Dead Apostle Ancestor Merem Solomon who both have devil familiars that act as their limbs. But in order for these devils 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 devil a name that easily as it would be the same as admitting to the World that they are completely independent devils.
- Main article: Shade
Devil 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 devil 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 devil 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 devil's strength. Because the devil's shape is one that is biologically impossible, this transformation process usually results in the death of the host before the devil can fully mature. Still, in some rare cases there are those who are able to survive the process and become fully matured Devils 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 devil before it can develop itself, for a fully matured devil 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 devil during the early stages of possession. And devils 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 devil 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 devil, so in these cases there is no choice but to call in an Executors to destroy both the host and the devil 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 “devil 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 devil 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.
These Demons are born from Gaia with the law of nature. 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 Devils (真性悪魔, Shinsei Akuma?) or Incarnated Demons (受肉した魔, Junikushita Ma?), as they differ from the imaginary devils born from human desires. They are creations of God that were Devils even before being named so by people. Born to be Devils 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 magical energy. 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.
True Devils are the greatest obstacle to mankind. If God is all-knowing and all-powerful, the devil is a being of human knowledge and powerlessness. It is the reachable but utterly incomprehensible abyss. In Fate/stay night, the shown devils 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 normal devil 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 devils, but this is called demonization so they are not True Devils. 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 Devil. In the true route of Fate/Extra CCC, Kiara Sessyoin underwent the transformation into a devil, but failed to reach the level of True Devils.
Though the terms "devil" 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. 
The Oni Kind (鬼種, Oni Shu?) are Demons born from nature that are symbolized by inferno and scorching heat. Two prominent members of the species are Shuten Douji and Ibaraki Douji. They declined as human civilization advanced, and are now all but extinct.
In the past of Japan, many families wanted to crossbreed with these creatures. 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.
Those with thick blood are often overwhelmed by their blood with Inversion Impulses, and those that maximize the blood are called Crimson Red Vermillion (紅赤朱, Kurenai Seki Shu?). Also known as Ancestry Return, they are enveloped by an illusion-like fog and their demon blood overwhelms and consumes their human blood. Those that become Crimson Red Vermillion are never able to return to humanity again. Having a thick demon blood, Akiha Tohno has a 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.
 Comptiq 2007-05 issue - Fate Dojo Q & A:
Q: Who's the top three strongest characters?
Q: In Nasu's work (Kara no Kyoukai, Tsukihime, Fate, DDD) Who's the top three strongest characters?
A: Magical Amber, Neco Arc, and Tiger. Well, not really.
Seriously, it's Arcueid, "Ryougi Shiki", and primordial demons.(Not counting Servants)
 Fate/EXTRA material - Encyclopedia: True Demon [Circumstances], p.190
True Demon [Circumstances]
Even in the Extra world, the concept of the demon is treated as 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 stay night, the only 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, but that's another story).
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 of their own doing cease being human and end up being categorized as demons, but this is called demonization so they are not True Demons.
Whether it's possession or transformation of one's doing, there has yet to be one successful case of a phenomenon that has created a True Demon as of 2030.
The names demons, demonic incarnation are used but they are no different than higher dimensional beings from a different civilization, different planet.
After all, they've ceased being "people" in our perceived universe.
Their nature is close to the evil gods of a certain fictional mythology popular in the 20th century.
A certain person reborn in the True Route of CCC, despite reaching this region, fell from the seat of God because of a trivial mistake.
 Tsukihime Dokuhon PlusPeriod - TSUKIHIME Dictionary: Crimson Red Vermilion [Term], p.178
Crimson Red Vermilion [Term]
The name for when those with inhuman heritage have drawn out their sleeping blood to the maximum possible extent. Once they reach this state, they are wreathed in a mirage-like haze and can never return to being human.
The name Crimson Red Vermilion is unique to the families ruled over by the Tohno household. Elsewhere, it is known as Ancestral Return.
As one with particularly tainted blood, the possibility that Akiha will become a Crimson Red Vermilion is high.
In Tsukihime, when her hair turned deep crimson, she was on the verge of this, though somehow she managed to bring her human and oni blood under control. Of course, rather than returning to how she was before, it's more accurate to say she switched to her "human" self while remaining close to the state Crimson Red Vermilion.
 Melty Blood Act Cadenza PS2 Manual - Dictionary: Crimson Red Vermillion [Term], p.057
Crimson Red Vermillion [Term]
Crimson Red Vermillion.
The name for when those with inhuman heritage have drawn out their sleeping blood to the maximum possible extent.
The name Crimson Red Vermillion is unique to the families ruled over by the Tohno household. Elsewhere, it is known as Ancestral Return.
As one with particularly tainted blood, the possibility that Akiha will become a Crimson Red Vermillion is high.
You can catch a glimpse of this during Tohno Akiha and Kishima Kouma's Arc Drives. Also, though Kouma is the closest to attaining it at the current stage, Akiha is actually the one with more aptitude for it.
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