A promise is a promise:
The Cainite Heresy: More frequently known as just "the Cainites", the Heresy is an old mystical order dedicated to the total eradication of the "Marked of Cain"-ie, vampires, and to a lesser extent all other undead. Composed almost exclusively of people that would frighten Alexander Anderson in their fanaticism, the Cainites are a non-centralized, flexible organization who's main commonality between cells is their use of a unique form of blood magic called the Rites of Denial, in which the blood of five Cainites (and only Cainites-how a political affiliation does this is anyone's guess) are mixed together in a specially prepared phial to create a solution that can be used in a variety of anti-undead spells. Their ideology is similarly flexible-quite simply, they teach that vampires are the ultimate source of all evils in the world, and that only after the extinction of their kind can the sin of the First Murderer (Cain, in case you're wondering) can be redeemed and the world turned into a true paradise. Other than that, their only real distinguishing mark is likely the last question vampire is ever asked; "Who is Cain?", said just before a blitzkrieg ambush on the bloodsucker in question, and usually a successful one.
All of this helps obscure the fact that, when it comes down to it, the Cainites have absolutely no bugger-all idea how their magic works, or even how they are tipped off to the location and plots of vampires in the first place; Rather, they are guided to their targets and given new Rites via mail from the mysterious Sources, who also manufacture the ritual phials. Given that a Cainite has to be a diehard zealot even before being considered a potential member, any potential doubts are quickly quashed out-which is a good thing for the Sources, who are actually themselves vampires under the aegis of VII (it's a Roman numeral, not a word). An enigmatic group to say the least, VII seems to despise all other vampires almost as much as the Cainites for unknown reasons-and reasons that will likely remain unknown, given how they have learned a way to make their minds literally impenetrable, able to resist all forms of interrogation and mind-reading. Theories vary on what they are-the dominant three are that they are descendants of Gomorrah sworn to avenge their lost kingdom, with powers derived from the disgraced demon prince Shaddad (the Akhud), an entirely new strain of vampire created entirely separately from Dracula after a bloody feud with the Vampire Nation in Russia (the Betrayed), or the hosts of a sort of "organic computer virus" created by a radical member of the Lancea Sanctum gone horribly right (the Sleepers).
Les Mysteries: Less of an organization and more of a class of hunter with identical powers, the Les Mysteries are, in theory, people chosen to be emissaries to and from the Imagineria, but in practice, end up being it's stable of monster hunters. The result of a pact in which a person agrees to serve the interests of the Imagineria as a whole (disparate and vague those may be), the Les Mysteries gain the ability to temporarily serve as hosts for it's spirits, gaining their powers in return for serving their interests (the name for the group comes from the Rites du Cheval's similarity to many voodoo rituals). Generally, their duties involve spreading the belief in the supernatural and extraordinary, along with promoting the molestation of the Imagineria by those who seek to abuse it, thus promoting it's "ecology" so to speak. Unfortunately, their efforts to mold the spirit world to their liking often irks werewolves who resent a human's idea of what the ideal Imagineria should look like, thus leading to a cold war between the two groups.
Thanks to being a cabal in the very loosest sense of the word, the Les Mysteries like to divide themselves into various "sects", based around the way the individual members see their powers. The dominant, unnamed faction (often just called bokors) filter their power through the lens of the Voodun that gave the spirit emissaries their name-they are simply hougans and mambos who have a closer relationship to the Imagineria then most. The Apostolic Pentecostal Church based in upstate New York, formerly a particularly liberal enclave of the Long Night, believe that their abilities were thrust upon them by God, to serve as gardeners of the spirit world in much the same way that man was meant to serve as the Lords of Earth in the material realm. The Starlight Children are the most interesting, as they view themselves in terms of New Age science; Rather than being shamans, the Children think of themselves as psionically awakened ambassadors from a multidimensional alien race, and that their mission is to heal the universal unconscious of savage and animalistic thoughts in preparation for humanity's evolution (which, given what the Imagineria is, may not be as nutty as it first appears).
It should be noted, though, that these are only the most prominent and peaceful; Given the scope of the Imagineria, there are a literally infinite amount of potential sects, and it is inevitable that there are a few that possess a creed of domination.
Knights of Saint George: An old Anglican order of philanthropists and humanitarians, the Knights of St. George are a fraternal organization dedicated to missionary work and the reconstruction of lives destroyed by supernatural events. According to their creed, the Knights are supposedly an order formed to combat evil sorcerers and dark magic, named after the Dragonslayer because it was discovered that excessive use of powerful spells summoned the dragon that he gave his life destroying. They themselves were charted after a witch who had infiltrated the court of Richard the Lionheart and laid a curse on him so that Henry the Young could reign over England alone. A group of valiant knights discovered the curse, and after a long quest discovered the lost Goetic Gospels, esoteric teachings of Christ meant as a way to counter supernatural power with nothing but the soul. Using the strange magics in the Gospels, they were able to expose the witch and break the hex, at which point they decided to form a fraternity meant to protect the innocent from the trepidations of sorcery....
...At least, that's what they'd like everyone to believe. They desperately want to believe it themselves, because then, at least, they can sleep at night.
In reality, the Goetic Gospels are the devising of the Faceless Angels, a pantheon of Entropic Masters embodying the concept of misplaced virtue and justifications for wicked deeds. The exorcisms and rituals of the Knights actually draw upon the Abyss to consume wonder and hope in the form of magic and superhuman powers-the principle is that by using the dark parts of their own minds to call archetypal "inner demons" and sic them on spells (actually any effect that hints at a better and grander future, including "hypertech"), a Knight is actually inviting the Abyss to strip meaning and purpose from the world, leaving only emptiness and ennui behind. The belief that this ultimately works to the benefit of the world is critical to this process-the gap between the motive and the effect creates the cognitive dissonance that the Jungian Shadow thrives in, and the Shadow is needed to call up the inner demons. Of course, lower-ranking Knights don't know this-the bottom of the order honestly believes in the cover story, and mid-ranking members are just told they serve eldritch orders of angels rather then God directly (which is also when their orders grow increasingly amoral-after all, it's not an omnibenevolent deity that's guiding them, is it?). It's only the most diehard, self-justifying members that are told of the true nature of the angels, those who can honestly look at their own hypocrisy and convince themselves that the means justify the ends.
Of course, this begs the question of how, exactly, the Knights have gotten away for it for so long without going insane. The answer is actually quite simple, and quite tragic; The Knights do not serve, because they worship the Angels-rather, the Angels are blackmailing the Knights. According the Goetic Gospels, magic and wonder break down the barrier between what is real and what is not, and that allows the Abyss-a place that is both real and unreal-to enter the world. Too much, and the Entropic Masters will be able to enter reality through it, and awaken the Darkness Beyond Time. By feeding the Angels a steady diet of magic, the Knights can satiate the hunger of the Darkness, and keep it placid enough to remain safely sealed. Sadly, the Faceless Angels are not complete liars-the wonder they eat through the Abyss is channeled to the Darkness, convincing at least a part of it's intellect that it is witnessing the final dissolution of the remains of the universe before the Light remakes it. If that happens to be dark magic and terrifying wonder, so be it-so long as it inspires strong emotion, even fear, it satisfies the Angels (although they prefer hope and positive magic). As a result of what they have to do to save the world, the average high-ranking Knight is a compacted ball of existential dread and depression wrapped in a facade of humility and gregariousness, his hate for his masters matched only by his hate for himself-a hate, ironically, which provides a bastion for his sanity against the otherworldly madness around him.
Of course, this arrangement is also rather fragile-if there was ever some way to show the Knights that there's another way to pacify the Darkness without feeding it wonder, every single member would turn on Angels and publicly renounce them and the Goetic Gospels; The main problem would be preventing the more suicidal ones from going out in an attempt to take the Angels down with them.