About D.Gray-man HALLOW
It is near the end of the fictional 19th century.
AKUMA was created from the magic formula where the soul was invoked by a weak heart mourning a tragedy, turning into a killing weapon.
The "Clan of Noah", led by the Millennium Earl, commands hordes of AKUMA and continues spreading fear and tragedy throughout the world. The only counterpart that can destroy AKUMA is the "Black Order", which is associated with the exorcists.