Demons

This species of unknown name, called Demons by the children, Man-Eaters by Mister, Monsters by the Goldy Pond Resistance and simply as Them by Sister Krone, is comprised of tall humanoids with monstrous appearances and are the main antagonists of the series. They are the beings that command the "Demon World" in Factory Farming and Premium Farms.

History
In the past, the demons have even been referred to as devils and gods. It is currently unknown how the "demons" got to Earth or if they were native to Earth at all. Anything after the year 2015, which is stated by Ray to be the last year of any updated book at the Grace Field House. Norman theorizes that they originally resembled bacteria, and began to eat and take on the traits of animals and eventually humans.

According to Sonju, as the "demons" preyed on ancient humans, some of the humans began to fight back. After countless deaths and wars, a promise was formed between the "demons" and the Ratri Clan. The human and demon realms were split in two, where neither was allowed to cross nor hunt each other. The Ratri Clan left an unknown amount of humans in the demon realm, stranded with no way to escape, to become livestock to the demons in order to keep peace in the human world. Thus the plantation system was formed via a loophole, as the demons were not hunting them.

Types of Demons
All five known varieties of demons bear different and unique appearances to distinguish each of them from one another. Below is a list of demons according to their race or class.

Farm Demons
Farm Demons are all tall beings that have a long, pointy face with two eyes above each other and two horns on top. They have a mouth with sharp canines and a long tongue. They have long arms, six fingers and sharp nails. They each have four limbs and walk on two legs.

Some demons have been seen with a pipe connected to their mask. It is currently unknown what purpose this is for, if it serves any purpose at all. All demons so far have also been seen wearing a black cape.

Tamed Demons
Tamed demons appear to be demons which bear the most resemblance to the man-eaters (with the exception of having four instead of six legs instead).

Heathens
Heathens appeared to have the most human-like looks compared to the other types of demons— they each have four limbs and walk on two legs. The heathens are also the only group of demons to have horns on their heads.

Just like the farm demons and poachers, the heathens are polydactyly, with six fingers on each hand and wear masks with holes for them to see through.

Heathens are specifically known for their religious beliefs of not eating humans, but that does not restrain them for doing so in future, as a heathen, Sonju, bears the desire to prey on humans again in future.

Poachers
Poachers appear to be the wealthiest kind of demons. They look and behave the same as Heathens, with the exception of wearing more prestigious attire. These demons also wear masks to protect their middle eye from any harm. Poachers appear to have 3 long fingers on each hand, have tall slender bodies and quick reflexes. They also seem to be really fast, showing this with their exceptional abilities and skills.

These demons buy or steal humans from the premium farms and hunt them in the secret reserve Goldy Pond. Aristocrat Bayon hunts for fun while Grand Duke Leuvis hunts for excitement. The demons eat human meat after each hunt, with the meat nicely chopped into pieces shaped like steak.

Wild Man-Eaters
Being the least human-like than their other counterparts, man-eaters crawl on six legs and have numerous eyes, and are completely unclothed. They also appear to be the largest of the species. A wild demon can be killed quite easily if a human is an experienced demon-killer like Emma, Ray, and Mister for example, making man-eaters weaker then their tamed counterparts.

Powers and Abilities
Anthropomorphism: For anthropomorphic demons, similar to humans, they can walk on two legs and speak the human language. Several anthropomorphic demons are often seen equipped with weapons, such as spears and daggers, which they use to hunt humans and even their own kind.

Language: Besides being able to speak the human language, the demons sometimes tend to communicate in their own native language.

Ear-piercing Scream: A wild demon can let out an ear-piercing scream to signal to its surrounding allies.

Invulnerability: Wild demons are also shown to be rather invulnerable to physical attacks unless they are aimed at their eyes, which is considered their weak spot. Similarly, during the fight, Emma, Ray, Nigel and Violet had with Leuvis, the team successfully shot and broke Leuvis' mask, but the demon himself remained unhurt.

Regeneration: Demons can heal any kind of (their) wounds, except their middle eye.

Assimilation: Demons can be consumed by other demons, resulting in the two minds of the demons merging and acting like one.

Enhanced Speed & Reflexes: Wild demons are shown to be physically adept (and possibly more so than their civilized counterparts) as they are capable of running, pouncing onto their prey, and climbing up trees at great speed.

Evolution: Demons are capable of rapidly evolving by obtaining the characteristics of whatever they consume, which is how many have gained sapience.

Weaknesses
Middle Eye: This is demons' only known lethal vulnerability. The anthropomorphic demons always wear protective masks that largely conceal their faces with the exception of an eyehole on their mask's top and bottom, respectively. However, their mask protectively hides their middle eye because once this eye is destroyed the demon dies.

Devolution: Demons must continue to eat in order to retain their characteristics. As a result of consuming low quality factory farm meat for 1000 years, the ability has deteriorated for inferior demons, and even a period of six months could cause them to lose intelligence and return to the wild. Demons can lose this weakness by ingesting the blood of Mujika or someone else who has done so.

Diet
While they largely consume humans from Premium Farms and Factory Farms, they apparently eat other foods like chicken, sausage, grapes, apples, bread, wine, and cats.

The Demons offer the premium-grade quality food to.

It is later revealed that Demons eat humans not simply because they enjoy it, but because they need to. Demons take on the characteristics of whatever they eat, which is why they vary in appearance, size, intelligence, and abilities (i.e., reasoning and thinking). This explains why some are more like predatory animals, and others are sentient humanoids. If a Demon goes too long without eating humans, it gradually loses its humanlike sentience as well as reassuming more animalistic forms.

Trivia

 * "Heathen" means someone who does not belong to a widely held religion as regarded by those who do. This might explain how heathens like Sonju and Mujika are only a minority out of the vast majority of demons that eat humans.
 * The wild demons/man-eaters are ranked 20th in the Popularity Poll that was announced in Chapter 88 of the Manga.