What is the difference between a priesthood of an evil deity, and a cult? I like Bane as a deity, but his worshipers seem too open to be a "cult". While the religion of Cyric must be secretive, maybe that's just because Cyric used to be a thief.
Many things that are called cults in D&D don't involve deities. Many demonic/devilish cults, or Far Realm cults, for instance (the powers that they worship might be on par with deities though, even if the powers they grant are different). There might even be species-based cults. Yuan-ti secretly worship Zehir... but this is probably because they're yuan-ti, not because Zehir is specifically demanding secrecy.
One of the coolest examples I've seen isn't D&D. It's Warhammer Fantasy. In that setting, there's no racial distinction between high and dark elves, just cultural. Many dark elves worship Slaanesh, god of excess, and they can infiltrate high elven society and "seduce" (literally or otherwise) high elves into "turning to the dark side". (This wouldn't work in D&D though. A drow isn't a high elf or eladrin, and there aren't any evil gods that elves and drow could worship in common.)
Come to think of it, the Sith are effectively a cult.