So You Need A Cult For Your RPG Game
Let’s dispense with the obvious: if you just want a bunch of crazy people in robes screaming about Hy’Fhen-Ator the Gore-Drenched while your player-characters (PCs) are busy burning down the Hy’Fhen-Ator’s Dark Temple, you don’t really need advice. Evil crazy cultists with a barking mad deity and an uncritical willingness to throw stupidly dangerous spells around: yes, that will be fine for many an adventure. And I don’t mean that sarcastically, either. Sometimes the PCs (particularly, the paladin-types) just need to really cut loose and Smite some Evil, right? After all, that’s why the players wrote them up in the first place.
But if you’re looking for something a little different, that’s fine, too.
We’ll assume, going in, that your cult is in fact evil. If it’s not evil then it’s probably at worst a weird, persecuted religious minority and those are not really fun to Smite anyway. But is the cult EVIL? Does it have a direct link to Something Bad?—That’s not the same as asking if it has access to magic, necessarily. In some game worlds baker’s guilds have access to magic. But is the cult actively making deals with demons, using the life force of babies to fuel its spells, or deliberately trying to destroy the world? Or are they ‘just’ doing the more mundane sorts of evils, like human trafficking, sedition, or building a criminal empire? This can be important if the PCs think that they need to bring enough firepower and urge for destruction to handle the first category, when in reality the cultists are in the second category—and are thus more likely to simply sublime in the blast wave. If nothing else, the local government might thus be inclined to object more if the destruction is too comprehensive.