Devils 「悪魔 Akuma」 are an ancient race of magical and malevolent beings that dwell in the underworld 「冥府 meifu」, a bleak world where the only amusement comes from tormenting each other and toying with the humans of the living world.
Normally, devils cannot appear in the living world on their own. A mage in the living world can use Forbidden Magic and a sacrifice to summon a devil and use some of its power. The amount of devil power a mage can wield is limited due to the gate between the living world and the underworld. A devil with no magic power can physically breach this border between worlds. Devils cannot wield or enter a grimoire until it has a five-leaf clover. Once they have a grimoire and a physical body, they can unleash their true forms and power in the living world.
Because devils come from the underworld, their mana is known to be malicious and sinister in nature, thus their magic attributes are particularly formidable and frightening. The only ways to counter devils' magic are with negative mana, Arcane Stage magic, Saint Stage magic, and other devils' magic.
A devil's heart is its most vital body part; as long as the heart remains intact, the devil could eventually recover, even if the rest of its body is destroyed. The heart is also the most resilient part of a devil, with high and highest-ranking devils' hearts surviving Ultimate Magic attacks and requiring Arcane or Saint Stage magic to destroy it. Additionally, its heart is not always located in the chest and could be shifted to different parts of the body at will.
The devils have developed a hierarchy in which their placements among the ranks are assigned at birth and are unable to be changed. A devil will never disobey one with a higher level than their own.  The known devil ranks are as follows:
- Highest-ranking Devils 「最上位悪魔 Saijōi Akuma, Viz: "Supreme Devils"」
- High-ranking Devils 「上位悪魔 Jōi Akuma」
- Mid-ranking Devils 「中位悪魔 Chūi Akuma」
- Low-ranking Devils 「下位悪魔 Kai Akuma」
Low-ranking devils can be defeated by hitting them with large amounts of mana. Mid-ranking and higher, however, require Arcane Stage mages. The ten devils connected to the Tree of Qliphoth are among the highest ranked and strongest devils.
The common defining characteristic of devils are their horns, which all devils boast. In general, the higher ranking the devil, the longer and more impressive horns they tend to have. Low-ranking devils are noticeably larger than humans and have rather bizarre and monstrous and beastly physiologies, like multiple eyes, tentacles, and bug-like mandibles. Mid-ranking devils are generally at least vaguely humanoid in appearance, though often with distorted bodily proportions and monstrous facial features. High and highest-ranking devils bear the closest resemblance to humans with slender, relatively proportionate figures but have pointed teeth, long, thin tails, and bat-like wings on their backs.
The underworld is divided into seven layers and is ruled over by three highest-ranking devils, who manipulate gravity, time, and space and reside at the lowest level. Each level houses devils of various ranks, with the highest-ranking devils dominating all those on their level. Because the underworld is so bleak and boring, higher-ranked devils keep themselves entertained by torturing those of lower ranks, while the lower ranked devils torment those who are even lower.
- For a full list of known devil hosts, see Category:Devil Hosts.
Devil hosts 「悪魔憑き Akumatsuki」, or devil-possessed, are humans who have direct access to the powers of devils. The host gains an immense boost in magic power and the ability to use the devil's magic attribute in addition to their own. When the host draws on their devil's power, their body takes on devil-like physical characteristics, such as horns, elongated canine teeth, pitch black skin, clawed fingers and toes, bat-like wings, and a long thin tail. However, it is impossible for a human host to access 100% of their devil's power, unless a direct channel is opened between the living world and the underworld or the devil inhabits a five-leaf clover grimoire.
By agreeing to bargains with their devil, the host is granted access to greater portions of the devil's power; this comes at the cost of parts of their own body, which are taken over by the devil from then on. As such, the number of contracts for power is limited by the strength of the host's body. It is also possible for the host to give up their entire body and even their very soul in exchange for a devil heart, effectively transforming the host into a devil.
A devil host can also share some of their devil's power with their underlings. While these underlings cannot use as high a percentage of power as a devil host can, the boost to their magic power still makes them formidable mages.
A devil can manifest in the living world from the underworld through their host, but it requires the souls of three powerful mages of the same gender. Once those mages die, the devil in question will be able to manifest fully.
A devil host can undergo the Devil-Binding Ritual 「従魔の儀 Jūma no Gi」, in which they fight and subjugate their devil and make a formal contract with it to use its power as their own. The connection is forged through a relic linked to the devil, and through this connection, the devil can temporarily be physically manifested in the living world. The ritual takes place within a magic circle that suppresses the devil's power. If the relic is destroyed during the ritual, then the spell will end prematurely and the devil will be sent back to the underworld.
If the host wins the fight, the devil's neck is bound and staked to the floor, and at that time, the host is free to make any contract they desire, including total subjugation of the devil. With full command of the devil's power, the host can easily use Unite, a special transformation available to hosts who have undergone this ritual.
An alternative is a contract of equals, in which the devil retains autonomy. With this contract, matching markings appear on the wrists of the host and the devil and uniting is more difficult, requiring the host and devil to work together and synchronize their thoughts.
If the devil wins, however, it will take full control of the host's body and transform them into a grotesque monster. Hundreds of humans have attempted the ritual and failed, and their skulls decorate the chamber where Asta undergoes the ritual.
- In many real world religions and cultures, devils are personifications of evil.