There are some people who will do anything to get what they want. A few will actually barter with the Devil or at least some type of demon.
The Damned are humans who are willing to provide services for demons in return for power. They worship demons and their Dark Masters and perform all manner of obscene acts to curry favor with their hateful overlords. They are fairly rare, for most demons prefer to work with people who do not realize who they are dealing with. Those willing to sell their souls to the Devil are pretty much damned anyway. Demons prefer to tempt the holy and pure, the kind and generous, and turn their good qualities into ashes. Those who are willing to Damn themselves are held in great contempt by the fallen angels.
Some Damned gain a measure of power from their dark deeds. One of their main purposes is the corruption of the innocent, usually through the systematic abuse of the young and helpless. Such victims are not only scarred by the abuse, but they grow up disbelieving in the existence of Good and eventually start their own cycle of abuse and terror. To reward them for these acts demons grant the Damned a number of special abilities, some of them mockeries of the powers of the Inspired. Instead of the Strength of Ten the Damned may get Berserker Strength that has similar benefits, but which makes the benefactor become a homicidal maniac. Other Damned receive Gifted Powers similar to the Sight and Necromancy, but conditional on their continual servitude.
Most demon-worshippers are nothing but mundanes, however, pitifully deluded mortals who commit minor blasphemies and hope to get something in return. Some are insane psychopaths who perform all manner of atrocities in the name of Satan, although the responsibility for these actions is theirs alone. Even mundane cultists can be dangerous, however, for their members are often quite willing to kill, and because their hatred and sick faith may act as a barrier against Magic and other powers.