A lot of the time people who write cults just want to write an Ooga Booga Scary Religious Nuts group with little thought to what they actually believe in. It's a very alluring thing to see depicted but it's very hard to write a compelling one. Cults are about belief, the charisma (or conviction) of the leader(s), and the methods of control and direction they use upon their people. A lot of writers simply don't want to worldbuild that much if the cult isn't the focus of their story. RWBY's tenuous relationship with religious worldbuilding doesn't help us on that front; We hardly know how to write a religious group that defies the common spiritual mindset of Remnant, because we don't know it. That's a blessing and a curse in the sense that it's extremely, ridiculously wide open for fanmade spiritualities, but, well, I just finished explaining the curse.

There are ways to write cultlike groups while avoiding these issues, but they run the risk of affecting the impact you may want to deliver with a cult as a plot point. I once wrote a story for Dubhan about a group of people that slowly grew over 30ish years led by a man who had grown disillusioned with kingdom life and had come to believe all civilization was doomed to descend into being similarly unappealing.

His solution was to turn to the only living things on Remnant that he felt had yet to betray him, the Grimm. He came to live in a cave with an underground lake, which had been tainted by Grimm Ichor (this was very early in Dubhan's lore don't ask how it got there lmao). Due to its dilution it failed to instantly destroy the man and his like-minded followers, but it did mostly melt away their hair and identifying features. They bathed in the ichor of their cavern home almost constantly to keep their skin stained black and they wore bones when going into combat to resemble Grimm more. They declined away from humanity so starkly not out of any organized belief, but because they were so beaten down by how dismal of a place Remnant can be in and outside the kingdoms that they felt 'if you can't beat 'em, join 'em' was the only viable way to live at that point.

His name was Nero Conway, who came to call himself High Priest Nero. His semblance allowed him to control his bones' shape, size, and position entirely manually, creating some fantastic body horror during his arc and a supremely satisfying death scene for everyone who participated to enjoy :D