Demons

Demons (鬼,) are a source of magical powers and play a central role in the series. So far, a majority of them used to be vampires that later turned into demons under certain circumstances. They can be sealed in what the humans call Cursed Gear (鬼呪装備,) and are used by the Japanese Imperial Demon Army.

Physical Appearance
A demon's true form is usually of a vampire (red eyes, fangs, and pointed ears) of any age with some kind of horn on their heads. Upon drinking blood, their horns grow bigger and their finger and toenails become claw like. Those that do not have this appearance are Mahiru-no-yo (lacking the pointed ears of a vampire despite being one when turned into a demon, as well as any horn on her head) and Byakkomaru, who appears as a white tiger.

According to Noya, demons appear at the age when they were most pure and undiluted, when they felt that everything was possible. At first, it was thought that they would physically appear as a child or a teen below the age of twenty; however, with the appearances of Shigama Dōji, Raimeiki, and Gekkōin, all of them being former vampires in their adult years, Noya's claim is uncertain.

Behavior
Demons are creatures born from passionate desire, though according to Asuramaru, demons are not supposed to have emotions. This is elaborated on by Raimeiki when she states that demons are unable to remember their previous lives, hence they seek to feed off the greed and desires of humans. Their former lives as vampires could also be a factor since those that become vampires lose their emotions.

Vampire into a Demon
There seems to be various ways a vampire can turn into a demon. Ones revealed are:
 * Time - Krul Tepes says that all vampires eventually become demons. In chapter 20, Asuramaru tells Yūichirō Hyakuya that all vampires become demons in the end.
 * Lack of Blood Drinking - In chapter 11, Krul Tepes informs Mikaela Hyakuya that he will become a senseless, raving demon should he not drink blood, and Ferid Bathory tells Mika the same thing. Though in chapter 23, Kureto Hiragi, through an experiment by starving vampires and exposing them to sunlight, complains that most vampires do not become demons, and laments the difficulty of doing it with low-class vampires. This suggests that highly stressful conditions may turn a vampire into a demon, but no explicit explanations or examples have been shown on how this occurs.
 * Removing Their Heart - The third progenitor Lest Karr shows that vampires can turn into demons within seconds after having their hearts ripped out from their chests.

Human into a Demon
Full Article: Namanari

After a human comes into contact with the demon, the demon will begin to merge his or her personality with that of its host's. This forms the being known as a namanari. The original human personality is present but takes on the demon's characteristics, becoming full of dark desires. At first, this merged personality is full of primordial urges, such as lust, destruction, and violence. After that, the desires grow more complex and nuanced, and starts to affect the person's thinking.

Eventually, the infected persons may become a new demon completely.

Considering the fate of Mahiru and Shinoa Hīragi, one may be born with the trait inherited from a demon-possessed parent and gradually become a true demon. In this case, the transformation is enhanced at puberty until the person changes completely. One may also become a demon after contact with a demon, such as in the cases of Yūichirō Hyakuya and (temporarily) Yoichi Saotome. Another method is to infect a person with a demon, such as in the case of Guren Ichinose.

Should the demon overwhelm the person it comes into contact with, that person will begin turning into a namanari.

Known Demons
Note: When a vampire turns into a demon, he or she will traditionally have a new demon name. For example, Ashera Tepes became Asuramaru, and Mahiru Hiragi became Mahiru-no-yo.
 * Abaddon
 * Akahebi
 * Asuramaru
 * Byakkomaru
 * Chijiryū
 * Gekkōin
 * Genbushin
 * Kagutsuchi
 * Kakuze
 * Kiseki-o
 * Kujakumaru
 * Kukuri
 * Kuronagi
 * Kushitamahime
 * Mahiru-no-yo
 * Noya
 * Sankoujitsu
 * Shigama Dōji
 * Shikama Dōji
 * Raimeiki
 * Tenjiryū

As Power Source
One of the objectives of the Japanese Imperial Demon Army is to capture noble vampires, force them to morph into demons, and seal them to reside into weapons called Cursed Gear to be used to combat against enemy forces. It would appear that stronger vampires make stronger demons, which make stronger Cursed Gear.

Contract Ceremony
A soldier must first choose which weapon in which he or she prefers the most and step inside the ritual circle. Then the ceremony will start automatically. As soon as he or she lays their hand on the weapon, the human enters a trance. The soldier and demon battle within the person's mind, and the demon will use painful memories against its host. Once the human convinces the demon to give him/her power, the demon makes a contract. Demons feed off of desire, but they are weakened by love. The human must be careful to have plenty of desire, but to be able to reign it in as well, lest the demon take over. The only reason demons form this contracts in the first place is to have an opportunity at stealing their host's body. In return, demons allow their hosts to have immense power.

A host weak in willpower will be overcome, and a host without balance will likewise lose control. This is most prone to happening during ceremonies or advanced training. A demon who possesses its host turns the host into something called a man-eating demon. This demon controls the human's body and attempts to eliminate nearby humans that may cause it trouble.

After a demon becomes contracted to a human, that demon will continue to attempt possession by engaging the bearer in a battle of nightmares whenever he or she sleeps.

Advanced Training
After spending a period of time, typically six months, to acclimate to one's Cursed Gear, the bearer can move on to the next stage of training. If the bearer tries this too early, then the demon has greater odds of taking over. Due to the risk of going berserk, this must be done in a location a safe distance from the city, humans, vampires, or any place important.

First, the bearer activates his/her Cursed Gear to the maximum he/she can currently manage. After doing this, the bearer cuts himself/herself and commands the demon to drink his/her blood. The human enters a trance and collapses. This typically takes around twenty hours, and the human may go berserk at any time. The blood loosens the curse binding the demon. Once the human and demon make a serious connection, the human body jolts and shakes. Next, the demon must explain the rules of the human-made curse and contract that holds it prisoner. The curse loosens more. The human and demon wage a mental battle until one submits to the other.

The looser the curse, the greater the power the human can access, but the deeper the demon can dig into the human's heart and have an opportunity of taking over. One way to regain control is to think of one's loved ones.