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.

In case of a human, however, they are called Devils (魔人, Majin). Devils rarely occur, but are far stronger than a normal Demon. The Devils have become more prominent since Oliveira von Schtradius developed a process for turning humans into Devils.

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 Wolford, both Demons and Devils lost all of sense of reasoning.

The demonized hunters who serve Oliveira von Schtradius show they are still reasonable but desire to kill without much reason beyond it being how they express the anger and misery that caused their transformation.

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 thanks to the experiments of Oliveira von Schtradius.

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

Abilities
Demons are more powerful than their beast counterparts. Even weaker Demons are known to become exponentially more powerful once turned, requiring trained knights with the aid of experienced Magicians to take them down. Some are even strong enough to overwhelm human armies.

Devils, however, are in a completely different league. Displaying greatly enhanced strength, speed, resilience and Magic Power, enough to devastate the country and easily take out entire human kingdoms. Only exceptionally powerful Magicians like Merlin, Melinda, Shin and members of the Ultimate Magic Research Society have been capable of killing Devils.

Demon Classes
It is mentioned in the anime and manga, there are classes in describing the Demon's strength. It is implied that the classes from the weakest to the strongest are:

THREAT LEVEL : ''' ! ! ! ! ! ! ! ! ! ''' These are the weakest Demon, one example is the demonized wild boar in the combined training of Ultimate Magic Research Society and the Knight society, the others are demonized wolves and sheep, as long as they are not big animals. Even so, they have superior agility and strength.
 * REGULAR

THREAT LEVEL : ''' ! ! ! ! ! ! ! ! ! ''' Shown as the bigger, more ferocious demonized bigger animals than the regulars, DISASTER/CATASTHROPE class shown to be stronger than the regulars and their appearance are seen as a big threat . THREAT LEVEL : ''' ! ! ! ! ! ! ! ! ! ''' Shown as demonized ferocious big animals with bigger size than usual such as tiger in manga chapter 12 which has exceptional agility (only weak to close combat/physical attacks) and the lion which has exceptional strength (only weak to ranged attack). Shin Wolford asks the members of Ultimate Magician to defeat this class in order to prepare to deal with Demon invasions. Siegfried Marquez and Christina Hayden, the Diseum von Earlshide's guardian, was shocked at chapter 12 when Shin Wolford defeats this class instantly.
 * DISASTER/CATASTHROPE
 * CALAMITY

''===== Starting from here, the class's name are not official. They are fanmade and being used to categorize the strength of other demons which are implied stronger than the CALAMITY class ====='' THREAT LEVEL : ''' ! ! ! ! ! ! ! ! ! ''' Created by Oliveira von Schtradius as an experiment, can use offense Magic and can eventually destroy the school by exploding himself if not stopped. Nevertheless, Oliveira von Schtradius and Shin Wolford mentioned that he is weak in terms of power. One of the factor Shin Wolford recognized as the hero, is because he defeat the demonized Kurt von Ritzburg.
 * SCHTROM'S EXPERIMENT (DEMONIZED Kurt von Ritzburg)

THREAT LEVEL : ''' ! ! ! ! ! ! ! ! ! ''' Created by Oliveira von Schtradius as 'disposable pawn' by demonizing regular citizens or people. They retain their senses, can uses magical tool, can causes a massive destruction in a kingdom, and can overpower many regular army's soldiers there. The examples are those who invades the Swedes Kingdom in anime Episode 11.
 * HUMAN DEMON

THREAT LEVEL : ''' ! ! ! ! ! ! ! ! ! ''' Created by Oliveira von Schtradius from probably the experienced ones in combat or Magic. Just like HUMAN DEMON, but with more 'tidy' and 'captain' like outfit and more magical prowess. The examples are Zest, Lawrence, also the hunter demons. Starting from this class, all the Ultimate Magician's member (except Shin) cannot defeat these ELITE DEMONS in the manga chapter 24 and 25. In chapter 51, Merlin Wolford and Melinda Bowen encounter two ELITE DEMON and defeated them with flawless victory easily.
 * ELITE DEMON

THREAT LEVEL : ''' ! ! ! ! ! ! ! ! ! ''' It is seen in the ep 12 of the anime that Miria's combat and magical prowess is so high that all of the Ultimate Magician's member (except Shin Wolford) could not match her a combat or Magic. This is shown when Miria singlehandedly defeats Alice Corner, Lynn Hughes, Tony Freed, and Yuri Carlton (the members of Ultimate Magician) in the Swedes Kingdom. Zest actually asked her to help the other Demon's to retreat from the Swedes Kingdom in response of the great losses of the Demons there. Shin Wolford said in manga chapter 65 that she is good in terms of strength and Magic. She retains her senses.
 * Miria

THREAT LEVEL : ''' ! ! ! ! ! ! ! ! ! ''' The first appearing Demon in history who lost all of his human senses yet almost destroy Earlshide Kingdom singlehandedly. He got demonized because he was a professional wizard who failed to control a high class spell. It is stated in manga chapter 2, the strength, combat, and magical skill of the FIRST DEMON is so exceptional that Merlin Wolford and Melinda Bowen had to fought him hardly and win with damages on their clothes and faces.
 * FIRST DEMON

THREAT LEVEL : ''' ! ! ! ! ! ! ! ! !  Oliveira von Schtradius can CREATE other demons''' and the one who demonized Miria. Oliveira von Schtradius, with his 'incomparable' strength and still retaining his senses, is with a little doubt, the most powerful human Demon ever. It is mentioned in the manga chapter 18 that the ELITE DEMON got extremely shocked when somebody (Shin Wolford) can even hurt Oliveira von Schtradius considering Miria and the ELITE DEMON are adoring Oliveira von Schtradius's strength.
 * SCHTROM

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.
 * Once the source of their rage is gone, the Demons lose all sense of purpose; akin to depression.
 * Natural demonizing causes explosions. Forced demonizing causes no such thing. But the size of the explosion depends on the Magic Power the Demon has.
 * Given Oliveira von Schtradius's transformation was enough to destroy his property and vaporize the deceived villagers who killed his wife, it can theorized that if Shin fell down this path, his transformation would be similar in terms of destructive force.