Human beings whose appearances has completely changed into ones that resemble "Demons". They hide their faces behind masks that have a distinctive look for each clan, so in a sense, this mask is the real form of the Diabolus. Painting it with the bloods of their enemies, the mask literally grows for each and every one of its owner's sins, and once the Diabolus wearer completely covers their face with the mask, they completely lose their humanity in exchange for perpetual youth and immortality.
As either a result of their masks or their intentions, their bodies have been remade to resemble demons in a manner that allows them to efficiently steal the life-force of others, this kind of madness is an ideal example of the Black World.
Diabolus means "the Devil" in Latin. Also, most of the Diabolus is named after Latin words.