I'm not that familiar with the south of the Colonies, but Shadowism and the Apostles of the Abyss are two pretty big, active religions, both covering at least two realms (The Assassins and The Electorate of Minas Thalion for Shadowism, while Outer Tilog and The Dark Court of Gothica follow the AotA). I know Lukon and Portion have the Horde of Honor, but I don't know how active that religion is. Shadowism is pretty well covered on the wiki, AotA are Daimon worshipers/crazy (depending on how you play your character). I think Horde of Honor is meant to be Mongol styled ancestor worship. Maybe someone can answer better about it and elsewhere.
