Diabolism is the art of utilizing the demons and devils to perform esoteric feats. Those who utilize this form of magic exclusively are known as Diabolists and a gathering of diabolists is seen as a cabal.
Diabolism involves binding and using Demons and is heavily frowned upon as the darkest of all arts available to use because of the nature of demons. Practitioners of it are discriminated against and inquisitors actively hunt them down. It's stated that most diabolists die horrible deaths or join the firm, with a minority actually living long lives and dying clean deaths.
Some diabolists see themselves as a necessary evil, as someone must deal with the demons and devils roaming the world, while others are malicious and utilize the art for destructive means that can only be accomplished by using demons. Those who practice the art also accrue a heavy karmic debt with greater ease.