The Cabal

The Cabal was a group of xenos races determined to fight Chaos, mentioned in a number of the Horus Heresy novels, primarily Legion. At least one Eldar Autarch, at least two Kroot ("Spavined avian creatures"), two humans (and Perpetuals to boot), and possibly the Watchers in the Dark were included (they totally were), as well as nearly-extinct, intelligent atmospheric layers and a small, not-dickish omniglot insectoid. Legion also makes references to the "Old Kind," but it seems that the Perpetual saying that was merely calling the Eldar et. al. "older than humanity," not referring to a literal race. They would have invited the Emperor, but he would rather kill all xenos than work with any of them, as he really is that much of a bloodthirsty bastard.

They are the group responsible for Alpharius' (and Omegon's) treachery (or LOYALTY?). Using the Acuity, a form of divination based on Eldar farseeing, they told them that they had two options:

• 1: The Emperor wins and the galaxy is plunged into 10,000 years of stagnation, during which time Chaos will grow steadily more powerful, eventually consuming the galaxy and all living things in it.
  • This option suggests that the Emperor's victory would have resulted in it playing out as it did in canon, when it only went that way partly due to the Cabal's interference. If they had helped or done nothing at all, things might have gone a different way and we might not have gotten have the grimdark galaxy we have today. This lie is partly justified in that despite their claims to the contrary, the Cabal are ignorant of the nature of Chaos, and the majority of the Cabal considered humanity under the Emperor's rule a threat to themselves (and their races). They may have chosen not to aid humanity outright due to distrusting their plans for what would happen after Chaos was dealt with, after all it's unlikely any of them were willing to have their races reduced to slave states (assuming the Emperor did not decide they would be better off exterminated).
• 2: The Emperor dies, Horus wins, and the galaxy is plunged into Chaos. However, Horus's guilt over having killed the Emperor would lead him to self-destructively trying to destroy the human race, and the Chaos Gods, having gorged themselves on human emotions, would collapse along with them; non-human life in the galaxy would survive, however.

Later events imply that neither Alpharius nor Omegon fully believed these, although they disagreed with what would happen. Either regarding these visions as false or at least trying to find another option, the twins have started working against each other, and therefore the Cabal consider this endeavor to have been a failed Gambit on their part and move towards other options.

This is further backed up in Vulkan Lives, where the dick starts trying to undermine the Cabal and save Vulkan, as he believes that an actual victory for the Emperor would be bad for the Chaos Gods, and if Vulkan is at the Siege of Terra, he would affect the outcome of Heresy in a way that would be beneficial to the overall Galaxy.

The Cabal also knows something about this, so they arrange for John Grammaticus to kill Vulkan using divine cast-off energy from the Emperor. However, this plan succeeds/is spoiled (depending on your viewpoint) when Vulkan is slain after being stabbed by the weapon, but it doesn't kill him permanently as they had anticipated.

According to The Beast Arises, the Cabal were wiped out during the Heresy. (Because Eldrad killed them all with help from Barthusa Narek in 'Old Earth'!) It also reveals that they had considered the Eldar's destruction an acceptable loss, meaning that the master manipulators had been manipulated despite the aforementioned Autarch insisting that it was Eldrad who was the race traitor.

Even in death, they still maintained that the Acuity was more accurate than Eldrad could ever hope to be. He finds this sentiment amusing, especially since it didn't let them foresee their own destruction. Not to mention the dumb fucks made the colossal mistake of thinking they could out-predict a group of practically omniscient entities, including the one known for both its interest in precognition and for its being the greatest schemer in the galaxy. Ultimately, it can be said that their attempts at fighting Chaos ultimately made the Ruinous Powers stronger instead (and knowing Tzeentch, that was all part of one of its plans).

TLDR; the Cabal tried to play dice with the gods. It went about as well as you'd expect.