Handy options for evil cults/organizations

[Nettlekid]

Just trying to brainstorm some kinds of evil organizations that could be used as campaign enemies. Ideally it would be a group that can have a number of tiers of members and spheres of influence, so it can keep reappearing and prove a legitimate challenge at any level without being forced to over- or under-level. More generally, can people suggest categories of monsters that are readily designed to be "a band of bad guys"?

Some examples of the trend I'm looking for are:
Illithids (especially the Mind Flayers of Thoon, with their many henchmen and creations).
Vampire cult (Dominated Commoners -> Spawn -> Vampires -> Vampire Lord)
Yuan-Ti (with their many mutations)
Any number of Devils or Demons (and to a lesser extent Inevitables and Slaadi)
Dragons possibly, with Draconic creatures as the lower ranking members.
Lycanthropes, maybe? A cult of Lycanthropes?
Kythons? Don't know much about them.

What I'm not looking for is just a band of Goblins or whatever with many different class combinations to make a large organization of builds. I want the organization built into the species in question.

[Âmesang]

My first thought was the Scarlet Brotherhood from WORLD OF GREYHAWK® who have an entire (2e) book dedicated to them (link (https://en.wikipedia.org/wiki/The_Scarlet_Brotherhood)). They're a tiered organization of pureblooded Suel humans seeking to reestablish the lost "glory" of their ancestors, the Suel Imperium, lead by monks and followed by assassins and thieves/rogues who have utter disdain for every other race, human or otherwise.

Basically they're Kung Fu Nazis. :smalltongue:

Though even the Brotherhood isn't perfect for while the majority worship Suel deities (Wee Jas, Kord, etc.), there's a splinter group, the "Black Brotherhood," who worships the fallen god of madness and entropy, Tharizdun. So that internal strife could be manipulated by the player characters, I'd imagine; help tear 'em apart from the inside.

The Brotherhood sneaks their agents into various courts and kingdoms, usually to spy or, in the case of the Holds of the Sea Princes, assassination; a blessing to a large port city named Sasserine.

If you have the Complete Arcane you can certainly make use of the Suel arcanamach prestige class.