The forces of darkness always leave a trail of tragedy and disaster wherever they pass. Be it demons from the Abyss or devils from Hell, these creatures lack any mercy or empathy and seek only the destruction or enslaving of any mortal life. Those lucky enough to survive an encounter with these fiends, are marked forever by the traumatic events, often driven crazy by the memories of everything they lost and nightmares that linger in their heads. A few though will focus the pain and suffering into a raging hate against these evil spawns, and devote themselves to hunt and eliminate these creatures at any cost. After all, they now have little or nothing to lose.
The Fiend Hunter class is a great addition to your Rage of Demons campaign, as this class focus is on hunting demons and other fiends.