Demons

Demons are the source of magical powers. They used to be people, who were later into demons under certain circumstances. They can be sealed in what the humans call Cursed Gear.

Mass-Produced Cursed Gear
Mass-produced Cursed Gear, which Yuichiro Hyakuya uses in chapter 2, is a much weaker form of Cursed Gear. 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
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
Usually, the power of demons can be sealed inside a small object, which are easy to carry around. Weapons with the power to cast blessings or curses can usually shrink. Shinoa's and Yoichi's weapons belong to this category.

Possession
However, as Shinoa explained, if the demons malice is too high, it's very ferocious and won't go disappear so easily. They are also said to be the type that will try to possess its user and its very common for them to engage in close combat to display it's might. Weapons like Yu's, Guren's and Kimizuki's cannot shrink like other weapons because they are in this category. Shinoa calls these weapons 'possession' types.

Black Demon Series
They are first rank 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.

Yaksha
They are second rank demons. Like the Black Demon series, many of them are aggressive, while others specialize in power amplification and weapon enchantment.

Bodhisattva
They are third rank demons. Like the previous demon classes, many of them are aggressive, while others specialize in power amplification and weapon enchantment.

Wisdom Kings
A lower ranked demon.