Devils are generally large and humanoid in appearance with pointed teeth, horns, long thin tails, and bat-like wings on their backs. Other devils have more bizarre and monstrous physiologies, like multiple eyes, tentacles, and bug-like mandibles.
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. Only negative mana, other devils' powers, and arcane magic can counter devils' magic.
Normally, devils cannot appear in the living world on their own. However, a mage in the living world can use Forbidden Magic and a sacrifice to summon a devil and use its power. A devil with no magic power can physically breach the 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.
Devils are born 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.
The devils have developed a caste system, and their placements among the ranks are assigned at birth and unable to be changed. Those of lower ranks are considered weaker and so are constantly tortured by higher-ranked devils. The devils connected to the Tree of Qliphoth are among the highest ranking and strongest devils.
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. 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 their own body. As such, the number of contracts for power is limited by the strength of the host's body.
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. However, if the devil wins the fight, 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.
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.
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.