For whatever warped reason, I hear circa 1987 L.L. Cool J saying the title of this post.
I also think of Ian Astbury:
But that's not what's going on here. Flourishing in the back waters and alleyways of this world are cults -- groups of devoted, often fanatical individuals who have dedicated themselves to the worship of something esoteric, strange, or downright dark and dangerous.
Cults and sorecerers are often intertwined, as sorcerers may get their power from consorting with demons or other supernatural entities (genii). Given the presence of an ancient and destroyed civilization (which I've decided, tentatively, to call Numitor), there are cults who worship the old, forgotten gods of this civilization. Also given the swords and sorcery vibe that I'm hoping permeates the setting, there has to be some snake or reptile cult that engages in human sacrifice, dark magic, and other unpleasantness designed to bring about the return of His Dark Majesty to corrupt and destroy the universe. That just has to be in there!
Although here's an idea that just popped into my head -- some sort of mashup between a Cult of Set and Jormungandr-worshiping Viking types who seek the destruction of the world. Campaign-wise, I'm usually not big on "save the world" sorts of games, but such a cult could be an interesting antagonist even at a minor level.
Not all cults have to be evil, of course. Some could just be hertectical according to the dominant religions. There could be an Asharite Cult who believes there are actually 6 points in the Crown of Stars, for example.
What other cults could there be?
(Of course you realize there is now some cult leader out in this setting named Astbury. His consort is a mystical woman who can control fire. No, he does not live in a sonic temple. That would just be silly).