Demons are a race of sentient beings found mostly within the Underworld. These beings are often described as being malicious towards Humans, in contrast to Angels in the Overworld, though some cases had been present of Demons appearing within the Surface World, and even the Overworld (ex. Sombra).
Demons come in a high variety of shapes and forms, most of which are typically larger than average humans. Most of these forms hold little resemblance to humans, and instead take on the form of other animals, or an entirely different being altogether. While demons do take on many forms, all demons can be distinguished due to their reactions to items laced in holy water, which Korosensei describes having a similar chemical reaction to a human being touching acid.
Demons typically hold little respect towards humans, either seeing them as a food source or as sources of entertainment moreover. A demon's diet is as varied as other beings, though it consists of flesh as a primary source, though demons will also consume a human's soul for sustenance. This treatment is also extended to locals of the Underworld that aren't demons, such as Homunculus.
Demons are ranked in their threats to the Surface World in varying classes. The type of class an individual is given depends on their danger levels and how much damage one could cause:
- Tiger - this rank is for threats to a large number of people, this class is for most demons encountered that hold some authority over demons under them.
- Demon - this rank is for higher ranking demons, and mostly placed on royalty in the Underworld. Demon-class demons are considered a threat to a entire city and it's people.
- Dragon - this is a rare rank to be given to any individual, and those with this level of danger are enough of a threat to effect the civilization on a global scale if left unchecked. The only one with this class as of now is Pluton.
- Wolf - this class of danger is arguably the most dangerous, as those under this class are on an unclear level. Wolves can fall under any of the previous classes depending on who they are.
Numerous types of demons are presented through the series, each one taking on their own forms and placements in the Underworld society:
Imps are a fairly low-ranking type of demon found within the Underworld. A few different types exist; the smallest ones green whereas the largest ones are yellow. Imps typically carry an assorted instrament with them. They aren't physically strong, but in the case of green imps, are capable of reviving the dead.
- Main Article: Hellgar
Hellhound is the overall term for dog-like demons found within the Underworld. Hellhounds, as the name implies, typically take on the form of a type of dog, and come in a variety of breeds.
- Houndooms are one example of Hellhounds often used as guard dogs to royalty. Their bites are said to be extremely venomous, and a bite from a houndoom will never heal.
- Houndours are a weaker variant of this species, and often follow the instructions of a Houndoom.
Hollows are a nocturnal species of Demonic entity that roams the lands of the Underworld. Hollows are the result of souls that were trapped in limbo for too long.
One notable type known is a Gillian; a building-sized being draped in black and wearing a pointed-nose mask. Envy describes them as the "wilder" version of curfew patrol, and will capture any being either human or demon that wanders too far from their settlements at night.
- Main article: Sombra
An Umbrum is a type of wolf-class shadow demon, and considerably a rare type of demon known to exist in the Underworld. They're mostly classified as demon-class, though some have exceeded to dragon-class levels of danger. Holy weaponry of B class and higher are recommended against a Umbrum.
Tengus are a type of aerial demon who mainly take on the forms of birds of prey. Most tengu seemingly follow under the lord of a being called the Great Tengu.