Demons

Demons (鬼,) are the 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.

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 Hīragi, not all demons were formerly vampires.

Mass-Produced Cursed Gear
Mass-produced Cursed Gear, which Yūichirō Hyakuya uses in chapter 2, is a much weaker form of Cursed Gear. Usually a Yaksha or Myo-O class weapon. These can increase a regular human's strength by 3-4 times. Seven humans with these weapons should be able to kill one unarmed vampire, but an entire unit of them would have a difficult time against even one vampire with a second-class weapon.

Cursed Gear
Invented by Mahiru Hīragi, these weapons are so powerful that they revolutionized the world and forced traditional magics to fall into disuse. A demon is sealed inside of these, and these can increase a normal person's standard ability by 7 fold. The top rank of Cursed Gear is the Black Demon Series, and anyone bearing one of them is capable of killing multiple vampires armed with second-class weapons. With lesser Cursed Gear series, the bearer can have an even one-on-one match against a vampire armed with a second-class weapon.

Manifestation
This gear has the convenient ability to remain in a smaller, more portable size or manifest into the full-sized weapon at will. These weapons may have the power to cast blessings or curses. Shinoa, Mitsuba, and Yoichi's weapons belong to this category.

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 and different ranges, and the type of ability varies.

Manifestation-type demons include Byakkomaru, Gekkōin, Shikama Dōji, 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. Weapons like Yu, Guren, and Kimizuki's cannot shrink like other weapons because they are in this category.

They are more well-balanced for offense and defense than manifestation-types because activating their particular capabilities does not leave their hosts vulnerable. They increase the physical strength and power of their user. They are typically limited to close-range combat for melee fighting.

Possession-type demons are not supposed to have the special abilities seen with manifestation-type demons.

Possession-type demons include Asuramaru, Kiseki-Ō, Mahiru-no-Yo, and Raimeki.

Black Demon Series / Bodhisattva
They are highest rank of demons and thus the most powerful. Many of them are aggressive, while others specialize in power amplification and weapon enchantment. There are very few people capable of wielding a Black Demon weapon, and anyone capable of doing so has been experimented on previously. Anyone bearing this rank of weapon can easily kill multiple vampires armed with second-class weapons.

Myo-O
The lowest rank of demons.

Contract Ceremony
A soldier must first choose which weapon 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. With contact, 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.