Originally, I was going to talk about cults and religions in fantasy, but I realized that it could paint the wrong picture.  Does Leo Kandrel lead a cult in War of Nytefall: Eradication?  Yes.  Is the cult connected to Durag the Sun God?  Yes.  Am I doing this question thing too often?  Probably.  Anyway, a cult doesn’t always have to be religious, but it tends to be that way in fantasy.  Here are some ways to work them into a story and possibly even drift away from the religious side of it.

1. It doesn’t have to be magic that makes everyone join the cult.  Sure, that can give the followers an easy out if they’re freed and they can go about their lives.  Yet, many times, people join a cult because it promises to answer questions and give them comfort in their lives.  They’re made to believe that the cult is their home and the only place where they are accepted.
2. One would assume that because there needs to be full faith in a cult that questions would be handled with drastic measures.  Violence or dragging the person away to be ‘fixed’ are common responses to drive home the ‘bad people’ factor.  It creates an army of mindless drones, which is how we tend to see cults.  Yet, you can get better results by having the leader answer questions in a way that brings the curious member even further into the fold.  These leaders are supposed to be smart and charming, so why not have them act that way in the face of doubt?
3. If you go the religious route for your fictional cult, you need to decide on if it’s an offshoot or the whole temple system.  You may feel that religions are already evil cults, so you don’t make any difference between the two.  That makes things easier, but you will have to show if religion has any influence on the lives of the heroes.  Makes no sense for the War God temples to be crazy cults, but one of the adventurers is a level-headed follower who is fairly casual about the whole thing.  This is why having there be the core following and groups of zealots that the core doesn’t even like can give you more flexibility.
4. While it isn’t a requirement that they have a central location, it would make things easier if they’re a major player in the story.  Of course, they can be a wandering band of cultists that descend on towns to gather new followers.  That’s a big reason for a local ruler to want them gone or a way for adventurers to stumble into them.  Yet, if they are going to have a large amount of reach and influence at any given moment then they may need a headquarters to operate out of.  This also reduces the chance of outsiders altering the perceptions of their followers, which is why you tend to hear of cults having compounds.
5. There should be a personal stake when it comes to the cult and your heroes.  It could be a relative is in there or they were once a member.  Another possibility is that the heroes stand for something that the cult is against.  For example, you could have a group composed of multiple races and the cult is against such things.  This is also a non-religious cult since they simply believe the fantasy races shouldn’t even be friends much less more.  Yeah, I’m saying you can go the racism route here.  Don’t act like it hadn’t already crossed your mind.
6. Most of the members should probably be weak and simple because they are the ones being controlled.  If you have a cult where every member is a powerful caster or a demonic warrior then there better be a massive spell keeping them in check.  The stronger the character, the less likely they are going to blindly follow someone else unless they have a personal code that explains it.  So, the leader and trusted group of agents should be the main threat from the group while the rest are fodder who may not even fight.
7. You may find yourself making a strict dogma for this cult and then running into a wall where their actions don’t fit your creation.  Well, there’s an easy way to fix that.  It’s called hypocrisy by their leadership.  Say it’s a cult of pacifism, but they end up killing people that break their laws.  This doesn’t make any sense to those of us outside of the cult, but you can have it make sense within.  The leader could do it themselves and say that they are taking the sin for the entire organization.  Maybe they’ve designated someone for that job.  It could be construed as saving them, so it isn’t really murder.  A final option is that the act is done without the followers knowing or in a way that they don’t realize the target is being killed.  Your logic doesn’t have to work as long as cult logic fits the pieces together.

You know, this post is a little scary in some ways.  I can see how these tips can be used to make a real cult.  Maybe?  I could be giving myself too much credit.