Demon

Demons (悪魔) are beings that originate from Gehenna, and are consistently found in conflict with Exorcists who aim to protect humans from their influence. Satan is considered to be the God of Demons due to his immense power and his rule of Gehenna.

Overview
Demons are beings that live in Gehenna, a world of nothingness that is parallel to the human world of Assiah, much like a mirror.

Demons come in a wide variety of forms, including animals, elementals, and corpses, among various other forms. Demons are also known to have special powers and physical attributes that are advantageous in combat.

A Demon has two major weaknesses: their tail and their heart. It is considered proper manners for a Demon to hide both their tail and their heart, as if the latter is destroyed, the Demon will die. Ironically, for a Demon to release its full power it must reveal its heart, which in turn makes them extremely vulnerable.

Demonic Possession
Ordinarily, Demons would be unable to make any sort of contact with the people of Assiah, however, by possessing whatever they can in Assiah, Demons can pass over into that world. This can range from humans, to plants, to the very dust particles in the air.

Their manner of possession varies; for instance, some Demons find their way into the hearts of humans by preying on their emotional instability. The host body must be strong enough to handle the power of the Demon possessing them, if the Demons power is too great for the host to handle, the body will begin to rapidly decompose.

Under normal circumstances, the Demon is in control of the host body, however the reverse can be put into practice as well. A human can force a Demon to possess them, allowing them to gain the Demons powers while also remaining in control of their own body. A human who does this is known as a Demon Eater. The rule that the host body must be strong enough to handle the Demons power still applies.

The only way to remove a Demon from its host and from Assiah is to exorcise it, which can be done by slaying it, reciting its fatal verse (provided one exists), or by allowing the Demon to remove any lingering attachments it has to the material world, as is the case with ghosts.

Elements
Nearly all demons belong to one of five main elements of demons: Fire, Rot, Air, Earth, and Water. The Demonic Elements Chart maps out the relationships of superiority among the elements. Following clockwise, the circle shows a cycle of overcoming; for example, Water overcomes Fire, and fire overcomes Rot. When two of the same element clash, the weaker gets absorbed by the stronger.

Elements on opposing sides of the circle are said to get along with each other; for example, Air with Water and Fire.

Mashō
A Mashō, or Temptaint, refers to any kind of wound or illness that is inflicted by a Demon.

Initially Demons cannot be perceived by humans, but once a person suffers a mashō, they become able to see Demons for the rest of their life. As such, it is a rite that all prospective Exorcists must go through, if they had not done so incidentally prior to becoming an Exorcist. Evidently, Rin did not have to undergo mashō because of his Demon heritage.

Nephilim
A Nephilim is an individual born from one human parent and one Demon parent. A Nephilim possesses Demon-like powers inherited from their Demon Parent. Over time, the Demon heritage is diluted due to the subsequent generations mating with humans, weakening the Demon powers.

A Nephilim is known to have a much longer lifespan than a regular human.

The Eight Demon Kings
The Eight Demon Kings are the children of Satan, and apart from Satan himself are the most powerful Demons in existence. Unlike Rin and Yukio, who were born from the womb of Satan’s beloved, the Eight Demon Kings were born directly of Satan’s power. Most Demons are classified by their affiliation with one of the Eight Demon Kings, and are referred to as being that King's "Kin" (眷属).

The Eight Kings (八候王), or Ba'al, are as follows:
 * 1) Lucifer, the King of Light (光の王), who has shown affiliation with Demons with luminous bodies. Such Demons include Seraphims. Lucifer is the strongest of the Eight Demon Kings.
 * 2) Samael, the King of Time (時の王), who has shown affiliation with Demons who are able to transcend the boundaries of space and time. Such Demons include Phantom Trains and Death. Samael is the second strongest of the Eight Demon Kings.
 * 3) Azazel, the King of Spirits (氣の王), who has shown affiliation with Demons who possess spectral bodies. Such Demons include Ghosts and Furfur. Azazel is the third strongest of the Eight Demon Kings.
 * 4) Iblis, the King of Fire (火の王), who has shown affiliation with Demons whose powers or nature involve flames. Such Demons include Peg Lanterns and Salamanders.
 * 5) Egyn, the King of Water (水の王), who has shown affiliation with Demons with liquid bodies, or who resemble aquatic animals. Such Demons include Reapers and Kraken.
 * 6) Astaroth, the King of Rot (腐の王), who has shown affiliation with Demons whose powers involve decay and/or illness, or who have bodies that are partially rotted. Such Demons include Coal Tars, Ghouls, Naberius, and the Impure King.
 * 7) Amaimon, the King of Earth (地の王), who has shown affiliation with Demons whose bodies possess plant and/or mineral attributes, or who have the attributes of land animals. Such Demons include Goblin, Dekalp, Greenman, and Bariyon. Amaimon is the seventh strongest of the Eight Demon Kings.
 * 8) Beelzebub, the King of Insects (昆の王), who has shown affiliation with Demons that resemble bugs. Such Demons include Chuchi.

Levels
Apart from Satan and the Eight Demon Kings, Demons are categorized into three different levels, Upper, Middle, and Lower, based on the strength they possess and the threat they pose.

Upper Level
Upper Level Demons are those who possess tremendous strength and pose a high threat to human society. If left unchecked, Upper Level Demons could bring about terrible chaos and destruction, such as the destruction of entire cities, or a widespread plague.

Upper Level Demons tend to be very large, larger than lesser ranked demons of the same type at least. In some cases they can be even bigger than a large building.

Middle Level
Middle Level Demons are those of average strength and pose noticeable threat to humans.

Middle Level Demons are of a medium size, around human size or somewhat larger.

Lower Level
Lower Level Demons are those that possess very little strength and pose relatively no threat. Ultimately, Lower Level Demons are more of an annoyance than an actual threat to humans, with a few exceptions.

Lower Level Demons tend to be of a small size, in some cases small enough to fit in the palm of someone’s hand.

The Emperors
Besides the Kings, there are "Emperors" (皇) called the Sol. After their incarnation, they joined with humans and engendered the Nephilim, in order to teach humans how to fight demons.
 * Shemihaza, the Emperor of Creation (創造皇)
 * Armumahel, the Emperor of Nothingness (虚無皇)

Galatea
A type of demon created by a Tamer. By forcing a demon to possess a certain object, they may animate said object, granting it sentience and supernatural power.