"Akuma" are a part of D. Gray-Man canon, and since they seem to be mentioned often enough, must deserve its own page.

Akuma are machines that are created with "sorrow, longing, and the Earl's magic". The Millennium Earl will visit a person who has recently lost a loved one and offer to bring them back to life. The person usually accepts the offer, and must call out to a magical skeleton the Earl has made. Using their voice, the soul is recalled from heaven and made into an akuma that must follow the Earl's orders. The akuma often kills the person that brought them back and wears their corpse.

Akuma are all but invincible except to the powers of innocence, which destroys the dark matter that they are made of. An non-canon assumption is that any type of holy powers would be able to free the soul, seeing as that is what innocence is. The Black Cross Organization specializes in destroying akuma and fighting against the Earl.