Demons (悪魔 Akuma) are beings that live in 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.
Demons are beings that live in Gehenna, a world of nothingness that faces Assiah like a mirror. 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 only way to remove a Demon 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.
Demons have two major weaknesses: their tails and their hearts. 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.
Most Demons are classified by their affiliation with one of the eight Demon Kings of Gehenna, and are referred to as being that King's "kin." The Eight Kings (八候王 Hachi-Kōō; lit. "Eight Assisting Kings"), or Ba'al (バール Bāru), are as follows:
- The King of Light (光の王 Hikari no Ō), who has shown affiliation with Demons such as Seraphims.
- The King of Time (時の王 Toki no Ō), who has shown affiliation with Demons such as Phantom Trains, and Death.
- The King of Spirits (氣の王 Ki no Ō), who has shown affiliation with Demons such as Ghosts and Furfur.
- The King of Fire (火の王 Hi no Ō), who has shown affiliation with Demons such as Peg Lanterns and Salamanders.
- The King of Water (水の王 Mizu no Ō), who has shown affiliation with Demons such as Reapers and Kraken.
- The King of Rot (腐の王 Kusa no Ō), who has shown affiliation with Demons such as Coal Tars, Ghouls, Naberius, and the Impure King.
- The King of Earth (地の王 Chi no Ō), who has shown affiliation with Demons such as the Goblin, Dekalp, Greenman, and Bariyon.
- The King of Insects (昆の王 Kon no Ō), who has shown affiliation with Demons such as the Chuchi.
Besides the Kings, there are "Emperors" (皇 Ō) called the Sol (
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.
The Demonic Elements Chart maps out the relationships among the five main elements of demons: Fire, Rot, Air, Earth, and Water. Following clockwise, the circle shows a cycle of overcoming; for example, Water overcomes Fire, which overcomes Rot. Elements on opposing sides of the circle are said to get along with each other; for example, Air with Water and Fire.