Demon

Demons (魔物, Mamono)(Not to be confused for the actual demon species that of course originates from hell but instead a classification that humans and animals go under when they go out of control and their eyes turn completely red) are animals whom without any intelligence and control over their strong mana, run rampant and become a dangerous creatures. However, in case of a human, they are called Devils (魔人, Majin). Devils rarely to occur, but are far stronger than a normal demon.

Appearance
The distinguishing feature of demons and devils are their red sclera.

Personality
Most demons and devils are creatures that are driven entirely by their uncontrollable desire to destroy everything in their path. According to Merlin, both demons and devils lost all of sense of reasoning.

Background
Demons appeared whenever an animal lost control of it's mana. It was initially thought to be impossible for a human to ever become a demon. This was notion was overturned by the emergence of the first ever devil. There are now devils who have retained their sanity and are able to think instead of being driven by pure instinct.

Chronology
Demons were first seen when Shin was hunting them in his youth.

Abilities
Demons are more powerful than their beast counterparts. Some of them are even strong enough to overwhelm human armies. Devils however are on a completely different league.

Trivia

 * Devils who have sufficient skill in controlling magic and retained their sanity can hide their red sclera by limiting the amount of mana they use.