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human
[Warhammer 40k] What is the cabal what is their origin and goal?

I've read that it's a secret organization againts chaos itself? Where do they come from what are some thing that they accomplished?
gpt
They are a secret organisation that have allegedly been around for longer than mankind has existed, made up of hundreds, if not thousands of xenos races from across the galaxy. Their ultimate goal is to once and for all wipe the existance of Chaos from the Empyrean. They do this by predicting the future and steering the intelligent races of the galaxy toward said goal.

They had a hand in the Horus Heresy, where they actually tried to steer it towards a future where Horus won, therefore extinguishing humanity from the galaxy, and as such, the Ruinous Powers would no longer have the psychic emanations of mankind to live off of, eventually disappearing completely. This failed however (obviously) and now mankind has fallen into stagnation, the Chaos gods only grow stronger. Because their machinations failed, they then predicted Chaos would overrun the galaxy sometime couple of thousand years after the end of M41.

Their plan to help Horus win involved them manipulating the Alpha Legion into fighting for Horus, despite their staunch loyalty to the Emperor, because it would be what is ultimately best for humanity. The Alpha Legion acceded to the request because they believed it is what the Emperor would have done if faced with a similar decision.

Another one of their plans was during the Istvaan Drop Site Massacre during the Horus Heresy. The Cabal came across the information that Corvus Corax and the Raven Guard would acquire a highly valuable secret asset of the Imperium, one that could turn the tide against Horus' Rebellion. samples of the original, undifferentiated Primarch gene-stock engineered by the Emperor for the original Primarch Project, and related information about its use and function. The Alpha Legion, with the help of the Cabal, stole the gene-stock, ending that threat to the Cabal's plans. To this day it is unknown what the Alpha Legion did with it. 

One notable member of the Cabal that most fans of the 40k lore should recognise was John Grammaticus, a genetically-engineered Perpetual who brought about Alpharius-Omegon's turn to the powers of Chaos.