It's not a PC-made faction, it's an official faction that was started and organised by the DM group.
And it might seem a bit high-fantasy, Forgotten Realms-y, but give it a chance to set in before you knock it. Ravenloft isn't full of brooding misfits alternating between defiling corpses, summoning demons and random murder, the Mists are known to regularly choose people with heroic potential (read: The Player Characters) in.
For now, think less 'Marble Hall, noble and bright shiny Paladins doing Just and Right for the Good of the People!', but instead a group of defiant heroes standing against the night, working from a cramped, run-down headquarters in the dingy slums, planning strikes into dark caves by the light of a flickering lamp.