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, 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. *BLAM!* He was wise enough to realise their plan was deeply flawed and would never bear fruit!

The Alpharius Gambit

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. Which we know is a blatant lie because the Emperor was obsessed with technological progress and because stagnation only positively effects Nurgle while being antithetical to the other three powers.
  • 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 almost assuredly would have gone a different way and we probably wouldn't have gotten 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). This tells us something about them, though. As the Emperor only purged species that chose to be a threat to humanity. Simply choose not to be and you’ll be ignored/conquered. In short, the Cabal were just unwitting pawns of Chaos.
• 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. This outcome doesn't look as retarded on face value, but with the benefit of hindsight one could see why this is still retarded on the part of the Cabal. Ignoring for a moment that one of the Chaos Gods exists BECAUSE of a non-human species, there's absolutely no assurances that the extinction of humanity would kill off the Chaos Gods. Based on existing lore, we know that the Laer worshipped Slaanesh, and the other Chaos Gods can still affect sentient alien races in some way if given the chance (which WILL present itself). There's no reason to assume the Chaos Gods couldn't and wouldn't feed off the emotions of other sentient life forms if humanity isn't available. Besides, the Emperor losing would have resulted in the galaxy being either turned into a giant Eye of Terror (wiping out all non-human life) or the Chaos humans exterminating/torturing/eating/raping all non-human life into extinction. Case in point: all Chaos humans are still shown to utterly hate aliens and seek to purge their existence.

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.

Originally, the Emperor’s victory prediction was that during the Emperor’s victory tour of the Imperium, a human would stab him with a Chaos artifact that would corrupt him. Eventually enough people pointed out that putting Chaos in the physical manifestation of anti-Chaos wouldn’t do Jack shit and that Chaos barring extremely powerful beings cannot even exist in his presence and so the artifact would have been rendered inert just being near him that Games Workshop retconned it to treat the canon result as the Emperor’s victory. Even though any retard can tell it was at best mutual destruction, not victory. It can’t even be said he saved the Imperium since the Imperium he built was essentially destroyed by his effective death.

Assassinating Vulkan

The Cabal's next move was to arrange for John Grammaticus to kill Vulkan using divine cast-off energy from the Emperor. According to the Cabal, only a Primarch was capable of using the fulgurite cast-off, so John was supposed to pass the artefact off to Konrad Curze and let him do it, but Eldrad co-opted Grammaticus and got him to do the stabbing instead. The end result being that Vulkan was slain after being stabbed by the weapon, but it doesn't kill him permanently as the Cabal had anticipated. Because the Cabal is too retarded to realize that you cannot kill a being made of the Emperor’s power using the Emperor’s power.

The reason for this is because the dick had started trying to undermine the Cabal and was attempting to save Vulkan. Eldrad believes that an actual victory for the Emperor would be bad for the Chaos Gods (a being Chaos is mortally terrified of and calls “The Anathema” winning against Chaos being a good thing!?), and if Vulkan is at the Siege of Terra, he would affect the outcome of the Heresy in a way that would be beneficial to the overall Galaxy. Eldrad later appeared at Mount Deathfire on Nocturne in the guise of an old man, and met a newly resurrected Vulkan, who had just forged an artefact called the Talisman of Seven Hammers. Eldrad directed Vulkan to the Webway so that he could reach Terra and fulfill his destiny. Apparently Vulkan gave Eldrad a drake tooth as a mark of trust between them.

After a short stop off with the Shattered Legions and destroying the remnants of his brother Ferrus Manus, Vulkan eventually made it through the webway to the Terran portion of it, which was being assailed by countless daemons. Vulkan used the last remnant of the fulgurite's power to destroy the army and make it to the Imperial Palace, where he discovered the Talisman of Seven Hammers acted as a kill-switch for the Golden Throne. In the event that Chaos won, the planet would be destroyed in a fiery inferno.

Considering that the Cabal wanted Terra to fall and for Horus to win; it can be understood why the installation of the kill-switch would be an outcome they wished to avoid. Plus having a regenerating demigod standing in defense of the Eternity Gate against the daemonic hordes in the Webway would probably make it a lot more difficult for the Emperor to lose too.

Eventual Fates

According to The Beast Arises, the Cabal were wiped out during the Heresy. Because shortly after sending Vulkan on his way, Eldrad killed them all with help from Barthusa Narek, a member of the Word Bearers who despised the direction his legion had taken and wanted Lorgar dead. It also reveals that the Cabal 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. Though due to the Eldar beliefs it could be the Autarch thought sacrificing the Eldar to destroy Chaos was their species’ goal (which is actually true, to be fair) but Eldrad believed defeating Chaos without that was preferred (also true).

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. A being who is, by the way, LITERALLY MADE OF EVERY THOUGHT THAT HAS EXISTED, DOES EXIST, AND WILL EXIST. 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.