Demons

Demons (鬼,) are a source of magical powers. They used to be vampires, who were 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.

Demons are creatures born from passionate desire, so they appear at the age when they were most pure and undiluted, when they felt that everything was possible. Demons shown so far appeared to be below the age of twenty with exceptions being Shikama Dōji and Gekkōin.

According to Asuramaru, demons are not supposed to have emotions.

In chapter 11, Krul Tepes informs Mikaela Hyakuya that he will become a senseless, raving demon should he not drink blood. In chapter 20, Asuramaru tells Yūichirō Hyakuya that all vampires become demons in the end. In chapter 23, Kureto Hīragi experiments on turning vampires into demons as well and laments the difficulty of doing it with low-class vampires. One of the objectives of the Japanese Imperial Demon Army is to capture noble vampires, force them to morph into demons, and curse them to reside in weapons they use for combat against enemy forces. It would appear that stronger vampires make stronger demons, which make stronger Cursed Gear.

Considering the fate of Mahiru and Shinoa Hīragi, it is not mandatory to first become a vampire in order to become a demon. 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.
 * Akahebi
 * Asuramaru
 * Byakkomaru
 * Gekkōin
 * Genbushin
 * Kagutsuchi
 * Kakuze
 * Kiseki-Ō
 * Kukuri
 * Kuronagi
 * Mahiru-no-yo
 * Noya
 * Shikama Dōji
 * Raimeiki
 * Tenjiryū

Turning a Vampire into a Demon
Krul Tepes says that all vampires eventually become demons. During an experiment, Kureto Hiragi complains that most vampires do not become demons.

Ferid Bathory tells Mika that going without blood for too long will turn him into a demon. In his experiment, Kureto was starving vampires and exposing them to sunlight. 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.

The third progenitor Lest Karr shows that vampires can turn into demons within seconds after having their hearts ripped out from their chests.

Turning a Human into a Demon
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.

Manifestation
Manifestation is the state in which the user lets his or her contracted demon come out and be able to use its special power. This drastically reduces the defensive abilities of the host since the demon moves elsewhere. These types of demons work best at different ranges, and the type of ability varies.

Manifestation-type demons include Byakkomaru, Gekkōin, Shikama Dōji, Genbushin, Kuronagi, Kakuze, and Tenjiryū.

Possession
If the demon's malice is too strong, it is ferocious and refuses to submit so easily. These demons are mean and aggressive. This is the type hell-bent on possessing its user, and it is common for them to engage in close combat to display their might.

Possession-type demons are not supposed to have the special abilities seen with manifestation-type demons. However, special cases have been seen with Asuramaru and Kiseki-o.

Possession-type demons include Asuramaru, Kiseki-o, Mahiru-no-yo, Noya, Kukuri, Kagutsuchi, and Raimeiki.

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 his/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.