Imagine being a citizen of the Imperium of Man. The first thing you'll ever know is that your individuality is worth jack shit. You are nothing more than a drone, a coin spent in an unimaginably large system, so that other coins can be spent elsewhere, continuing the cycle. No one arround you is ever expected to show you any kindness, respect you, or see you as anything less than another drone in the system. No matter how lenient your planet and government is, how strong an influence the Ecclesiarchy is where you live and how tough work you are expected to perform, this is the unfortunate and unchangeable truth that makes the Imperium what it is.
But say you've overheard some of the other drones whisper at your Administratum workplace in the upper hive. Apparently, there are these meetings, completely secret, but supposedly very invigorating. Your daily life here is hardly of note, maybe you should go. You had seen Dallia look at you with a funny expression, like she'd wondered something. Say you found the courage to forget what you've learned, and forget the punishment you and everyone even remotely associated with you would get if you were caught by the Arbites, and go to one of these meetings. Deep down in a forgotten and unused part of the Hive, you hear faint chanting. Already, your heavy legs feel stronger, your mind sharpens and your body seem to have a longing that was stamped out years ago... And when you leave, your body is as heavy as was it before you came there, but the longing remains. As time goes by, thoughts of work and duty fade, and you begin to wonder: Why are your own powers and skills suppressed? Think of all you could accomplish with them, if only you were given the chance to do so! And those decadent nobles in their high towers? Why do they get to have fun and relax? If you had the power, things would be very different: Very different indeed.
In one fateful meeting, your demagogue announces that their masters are finally arriving: Worshippers of what is true and fair are coming on ancient ships and plan to take the entire system and wrest it from the grip of the corrupt Imperium. That's your chance, the chance you'd been waiting for. Finally, they'll see what power you possess, what strength and intelligence they had all been neglecting. Doesn't matter what the others think: Only you are right to take what is yours, and no one is gonna stop you.
It's fucking payback time.
At least, that's what you think. In reality, you've just exchanged one form of miserable existence for another one that somehow manages to be even worse. But on the bright side, the Ruinous Powers will mess with your mind to the point you will neither notice nor care that your neighbor is adding your spouse's skull to the Khorne shrine and that the nice Daemonette you've been sleeping with is in the process of hollowing your soul out to make your body nice and comfy for the time she decides to take over it. This is of course assuming someone else doesn't grab you for a ritual sacrifice, or you don't starve to death due to the fact that demon cultists not being good administrators generally.
A Chaos Cult is a group of people worshiping the Ruinous Powers, either selectively or as a unified power. Cults are often comprised of a large mess of people who for some reason or other have forsaken their duty, and a demagogue, who is the spiritual and sometime martial leader of the Cult. The goal of a Cult varies wildly, from overthrowing governments or aid other heretical organizations, or simply to get a kick (or more than a kick, if you're into that sort of thing). Worshiping Chaos is different, depending on the god in question, but it often involves desecration of holy stuff, sacrifices, mass-suicide, writing and preaching heretical material, summoning of Daemons, and of course jaywalking - All in all, incredibly heretical stuff.
What most Cults don't know however is that cults attract more corruption, in form of warp anomalies, mutations and daemonic incursion. Even worse, if your corrupt actions light the Warp up well enough, it may very well attract a fleet of Chaos Space Marines. This might seem cool at first glance, but the Marines will likely not have your best interests in mind: As a matter of fact, many of them will want to put the entire system into the Warp and use you either as recruiting stock, as meatshields, or as sacrifices. Needless to say, none of those is a pleasant experience, although the former is at least a chance at starting the path to be a Daemon Prince.
Cults often worship one specific God, or sometimes a servant of that God, such as a Greater Daemon or a Daemon Prince, and the Cult is often different, depending on their allegiance. There are also a few other cult subtypes that aren't unique to any god in particular, but are used by all of them to some extent.
This has never been fully developed in the fluff but the White Dwarf Necromunda rules for everyone's favourite Chaotic psychopaths provides us with a structure:
- Each cult is lead by a Demagogue, often a Charismatic figure with an arsenal under the shrine. These guys tend to have swords, shotguns, axes, and occasionally will bear a mark of the corruption of the Chaos gods, manifesting itself in claws, or any variety of other mutations.
- Under the Demagogue, you have the Disciples and Cult Witch, with the Disciples carrying the "blessed" weapons, of a heavy/ flamer, or heavy stubber. These guys have perhaps the best aim. The Witch is a low level psyker.
- Cultists. Just Cultists.
Ever seen one of those crazy guys on the side of the street shouting at passerby about the coming apocalypse? You know, the kind of person with unkempt hair, a weird look in his eyes, and a crudely written sign announcing "THE END OF THE WORLD IS NEAR"? Get a whole bunch of people like him in one place, and you have an apocalyptic cult in a nutshell. As far as Chaos Cults go, they're pretty easy to understand- they're convinced that humanity's already doomed to be destroyed by Chaos no matter what it tries to do to stop the Ruinous Powers, so they try to lash out at civilization however they can to ensure the demise of mankind happens sooner rather than later.
You might think "if they're sure it's going to happen, why bother trying to speed it up?" The reason for this is simple: most members of an apocalyptic cult believe they will be on the winning side of the apocalypse, or their existence is so shitty that they can't wait for the Imperium to end on its own.
Much like several real world cults, these cults are centered around a single leader- a skilled demagogue, a powerful witch, or something similar. This leader will exploit the weakness of others for his own personal power or gratification, or the fulfilment of his own apocalyptic vision. While the rank and file are fanatically devoted to this leader, these cults' reliance on their leaders is a weakness as well as a strength; if the leader is slain, the cult will almost always collapse.
Cults to the Plaguefather are often comprised of people at the bottom of the Imperial hierarchy, such as mutants, slaves, poor and homeless, as these are often the ones to embrace the corrupting diseases of Nurgle, and because they are the most in need of the "love" that Nurgle so generously dispenses. They're also the ones most likely to succumb to his diseases as well. And with health care being a luxury in the Imperium, worshiping Nurgle proves to be the only way to stop the pain and agony of his poxes.
A Nurglite Cult is by no means an industrious one, though Nurgle approves of those who are willing to go the extra mile. Often, the mere act of worshiping Nurgle is more than enough to corrupt anything in the vicinity of the Cult and their members. More diligent cults go a step further in spreading corruption - they poison water and food supplies, release pathogens onto an unsuspecting public, and forcefully use otherwise useless people to create huge piles of rot and decay. When a Nurglite Cult grows sufficiently powerful, it often succumbs to its own plagues (being mortals and all): either the souls simply get collected by Nurgle and he makes them into Plaguebearers, or the entire Cult goes the way of Michael Jackson and zombifies itself.
Nurglite cults flourish in the largest of communities, generally hives. Monotony, melancholy, doldrums, and ennui need large populations to develop and grow strong.
The Blood God's cults are not Cults so much as they are raging bands of maniacs out for blood - their own or their enemies'. A Khornate Cult often breaks out as simple rebellions on particularly rough Imperial worlds where the leadership didn't get V's memo and overly abuses their workforce. What happens then is that every man, woman and child with the ability to use a weapon immediately vents their frustration on said leadership in ways that end with the oppressor's heads being forcibly separated from their bodies. But it doesn't stop there- after a while the would-be revolutionaries go completely nuts and start killing everyone and everything around them, up to and including each other.
Some Khornate cults skip the "rebellion" part completely and jump straight to the orgy of violence. These typically exist as offshoots of primitive Feral World religious traditions venerating Khorne in the guise of a native war god, and almost always involve blood sacrifices in some form or another. Usually, these are dealt with by Ecclesiarchal missionaries in the early stages of introducing the Imperial Creed, but they can sometimes go unnoticed long enough to reemerge and cause just as much damage as any other cult. In other cases, they simply kill random people on the streets, relying on the inherent violence within the dregs of Imperial society to cover their tracks and keep the Adeptus Arbites thinking that the dismembered corpses that keep showing up are just hive gangers going at each others' throats again.
Khornate Cults are dangerous as fuck to the Imperium, as of all the aligned cults, the Khornate cults are most likely to evolve into a terrifying fighting force that can rival several regiments of Imperial Guardsmen, to say nothing of their complete disregard for their own lives in their willingness to shed blood. As the adage goes, Khorne cares not where the blood flows from, only that it flows- and if they can't shed their enemies' blood, they'll settle for shedding their own blood instead. One example of a dangerous Khornate cult is the Blood Pact who managed to control all of the Sabbat Worlds at one point. By that point, they had become an organised independent government with a functioning and sustainable bureucracy and it took one of the biggest Imperial Crusades in millenia to expunge them.
Cults of Slaanesh are sick fucks. While not as large as the other cults, and usually not as common (as if), a Slaaneshi Cult can really fuck up any society at the core. The common stereotype of them is a bunch of jaded nobles looking for new ways to get off, but in theory anyone who wants a little pleasure in life can fall under the sway of a Slaanesh cult. The exact brand of sick fuckery that it fosters can vary widely, but regardless of what it is they fixate on it'll be as excessive and hedonistic as they can manage. Contrary to popular belief, this doesn't always equate to sex (that's so dark-goddamn vanilla); it can be anything from finding the most aesthetically pleasing pitch a person can scream at to mindlessly staring at a wall painted in a highly specific shade of purple, but still also can include more obvious things (like sex, drugs and ROCK) but turned up to "my dial's lowest setting is 11". It does, however, amount to obsession- a pleasure as seemingly harmless as listening to the crackling of wood burning in a fire can be warped to the point of monomania when someone starts tearing down houses to burn the wood that they're made of in order to gain that pleasure.
A good example of a non-sexualized Slaanesh-esque cult could be the first Bioshock game, in which you aid a drugged up artist with creating his magnum opus... Which is four pictures of his dead students held up by corpses of dancers and performers encapsulated in white plaster.
Tzeentchian cults are made of Just as Planned or Not as Planned in equal measures. Either way works for the Architect of Fate. They usually get to work sowing intrigue and dissent through dissemination of forbidden knowledge and concepts. They tend to be much harder to pinpoint than other cults as the quiet little reading club can just as easily be a cult front as the sorcerous fanatics summoning demons and mind raping civilians. As befits the Chaos god of hope and ambition, many such cults often camouflage themselves under laudable political and social causes before pulling out the real "Hope and Change" that Tzeentch is so well known for. Sometimes the cult's members don't even realize that they're part of a cult at all until they start turning into crimes against nature.
Often it's only the leadership of the cult who are devout Tzeenchian followers, while the rest are just their unwitting pawns, believing they are just venerating Emprah in some of His unusual aspects, or at least are Chaos Undivided cultists. The reason behind this is that as the god of ambition, Tzeentch cares much less about quantity of his followers compared to other gods and more about the quality thereof. This translates into a preference for recruiting mortals in a position of power- the better to use that power and any secrets they might know to his advantage. That's not to say he's snobbish about it, though- anyone with a penchant for deceit and cunning along with a heady dose of ambition has the potential to be a pawn too useful for the Great Conspirator to dispose of too quickly.
Contradictally and paradoxically, which is appropriate for Tzeentch, he also attracts those who dare to hope. In a sense, any rebellion can be a Tzeenchian rebellion because it is the desire for change and the hope of advancement that drives men to work against their masters. Throngs fall to Tzeench's sway just for the prospect of having a second loaf of bread.
Mystery Cults/Secret Societies
These cults center around the secrets and mysteries (hence the name) held within the highest ranks of the cult that the cult's power and authority stems from. In exchange for their loyalty and service to the aforementioned highest ranks, the cultists are promised the potential to learn and wield these secrets for themselves, which the cult guards carefully from any outsiders who might seek to learn them without pledging themselves to the cult in the process. The Warrior Lodges that formed within many of the Legiones Astartes near the end of the Great Crusade are a classic example of how a mystery cult operates, and their effects speak for themselves.
Technically, these "mysteries" don't necessarily have to be Chaotic in origin or even particularly religious in nature, and one could easily argue that the Adeptus Mechanicus is the most successful of all mystery cults. They certainly have the "mystery" part down, given their persistent refusal to tell anyone outside the cult how the average machine works.
Most death cults in the Imperium are technically part of the Imperial Creed, albeit rather loopy forms of it. However, Khornate Death Cults are not unheard of, as are some much weirder ones that either tend to be cannibals or have some very strange ideas about trying to resurrect the dead. The latter of these sects believe that the Emperor's plan is for mankind to follow Him into a blessed "immortality of the flesh." They even claim that it is possible through the use of utterly forbidden archeotech to free the Emperor from the Golden Throne to walk among His people, "dead-but-alive-everlasting" to quote the darkly renowned Credo Mortifex. Such cults are hated by just about everyone in the Ecclesiarchy for being a mockery of their teachings.
- Cultists - The mooks that make up the rank and file for Chaos cults.
- Warhammer 40,000/Tactics/Renegades And Heretics(8E) - Your means of creating your own tabletop legal Chaos Cult, courtesy of Forge World. Your demagogue's deity of choice can change which units are available. Not only that, but you can have radically different army lists based on what sort of HQ you take. Heretek Tech Priest? Have an army of bionic soldiers. Former Guard Colonel? You now have an army of storm-troopers. Slave Taskmaster? Congrats, you can take Ogryn as Troops choices.
- Warhammer_40,000/Tactics/Imperial_Militia_and_Cults_(30k) - While not specific to chaos cults alone, the list does allow you to create your own generic "warp cult" or horde of mutants. Is perhaps more customizable than renegades since you can mix-match army provenances to create a style that suits you, and allows you to proxy whatever models you feel like.
- Chaos in the Old World - A board game where chaos cultists are the primary unit, despite being mostly harmless (unless you play Khorne). Their purpose is for spreading corruption, which is the means for ruining nations and leveling your deity up.