These dark monsters are originated from Makai, and are spawn of Messiah and have existed since darkness first came into the human heart. As a result, being creatures that thrive on darkness, Horrors enter the human world to hunt and eat humans or simply torture them. At the beginning of the conflict, Makai Priests came into being to fend off the Horrors due to the threat that they pose. It was only after sealing Zedom that the Makai Order learned to develop Soul Metal and weaponize it in the form of powerful suits of armor that worn by Makai Knights who would be the primary fighting force against the Horrors.
Despite their nature as enemies of humanity, some recurrent gaining notoriety as they are named and documented, Horrors are not always targeted for elimination. At times, they are re-purposed by Makai Priests to serve various functions, such as for sealing of other more powerful Horrors, experimentation, or those like Sedinbale sealed to provide their vast knowledge on various Makai-related matters. Even Madōgu (魔導具 Madōgu) like Zaruba are created with Horrors that hold no enmity against humanity and have pledged their loyalty to the Makai order, the created items entrusted to certain Makai Knights of prestige.
Horrors, especially their normal variants called "Inga Horrors" (陰我ホラー Inga Horā, "Yin-Self Horrors") whom basic form is grotesque, black winged skeletal demon, enters the human world by being attracted to the darkness inside human beings that its kind feed on, using an object as a portal to travel from the Demon World. Those items, called Inga Gates, are objects with large amount of darkness from either playing a role in some sort of accumulated atrocity like mass murder, infused with dark energies, or a traumatic experience left unresolved. Regardless of how it is created, depending on its owner, an Inga Gate activates when it is touched by a person with inner darkness, with the emerging Horror turning that person or any other living thing nearby, into a host body. From there, the Horror "evolves" into a unique form based on the Gate they emerged from with personal tastes and feeding habits. In some cases, instead of taking complete control, an Inga Horror can form a symbiosis with the host to act out the human's dark desires.
Regardless, a human is dead the moment an Inga Horror possesses them and what remained of the host follows the Horror in death. Though rare, there are also some unusual Horrors that prefer to possess objects instead of humans or living things. Unlike human-possessing Inga Horrors, they do not seem to have any preference of prey, consuming whoever comes into close contact one way or another instead. The rarest of all Horrors are the ones taking forms of large, horrific beasts and have no need to possess anyone or anything. They are the least intelligent and are usually driven by an instinctive, indiscriminate urge to feed on humans only.
Three common things in Inga Horrors are:
- They usually hunt at night.
- Their blood can mark someone for death and increase desirability for other Horrors to consume the human soaked in thier blood.
- They speak in a language which can be understood by either someone educated in the Makai language or a potential host, usually adopting a human language upon possession.
There are telltale signs of a human converted into a Horror's host that can be used to differentiate them with normal humans, where the methods to see the signs are:
- Makai Tools: Makai tools, such as a Madou Lighter used to reveal the host's pale green eyes.
- Biruficated Jaw: Another, as revealed in Yami o Terasu Mono are bifurcated jaws that open up when a Horror, while in human form, is about to feed or screams in pain. Despite the jaws, some Horrors have different feeding methods that reflect their abilities and unique forms.