Akuma are demon machine-like creatures created by the Millenium Earl to wipe out the human race. They are created by a single shred of despair or grief. After someone dies, there is always someone grieving the loss of that person. That is when the Earl appears, he tells them they can bring the dead one back to life. If they agree, an Akuma's raw body appears before them, and with a single call of the deceased ones name, their soul comes forth into the Akuma's body, under the control of the Earl. The Millenium Earl then orders the new Akuma to kill the one who called the soul back, and after doing so steals the form of that person. All the humanity of the soul used for the Akuma ceases to exist at that point, and the soul is forever trapped and unable to control the body. Thus bringing life to a demon.

Allen Walker, the lead protagonist, and Exorcist of the Black Order, was tricked by the Earl into doing this for the one called Mana. However when Mana was ordered to kill him, Allen's Innocence activated, killing Mana, but not before Mana cursed Allen with the ability to see the trapped souls of those within an Akuma with his cursed left eye. Allen can now see the suffering soul hovering over the Akuma, and sense Akuma that are nearby. Unless an Akuma is destroyed by an anti-akuma weapon (which saves the soul attached), and it self-destructs, not only is the Akuma destroyed, but the soul within it as well and cannot be saved.

Throughout the course of the story Akuma have been shown to evolve, thus far the highest level being four. At level two, most Exorcists can deal with the Akuma's, however once the Akuma has become level three thats a different story. The group of Exorcists barely managed to defeat one level three, exhausting themselves to near death, except Allen who was finally able to unlock his Innocence full potential. A Level Four Akuma is so powerful that even an awakened Tyki could not handle it. A Level 4 Akuma is developed by sacraficing a Level 3 Akuma and tons of humans. At this point, only one Level 4 has been shown.