The Cabal is a group of mixed, ancient xeno species who claim to have battled the forces of the Primordial Annihilator for a time longer than the existence of evolved humanity. They are chiefly known for involving themselves in the development of the Horus Heresy by using their foreknowledge of Horus' actions to seemingly suborn Alpharius Omegon and his Legion to their cause before the Great Betrayal took place.
The Cabal and the Alpha Legion
Having studied the development of the primarchs over some decades, the Cabal were able to determine that the most important of them were the "oldest" (Horus) and the "youngest" (Alpharius Omegon). They foresaw that Horus would ignite a galactic-level civil war using the powers of Chaos, and claimed that what Horus 'would do', Alpharius would 'undo'. They also claimed that, while the Heresy was inevitable, the Alpha Legion was perfectly placed to control and direct it to one of the two possible outcomes.
Outcome One: Victory for Horus
The Cabal believed that, after the Heresy, a scintilla of the honourable being he once was would remain within Horus, eating at him and fueling a self-destructive sense of self-hatred and loathing that would see Horus fall on his subjects in an acceleration of the violence of the Heresy, creating civil strife amongst the traitor and Chaotic forces that would culminate in a final armageddon. The end result of Horus leading humanity would be total extermination of the human species within two or three generations...however, if this were so, the powers of Chaos, so heavily invested in humanity, would be extinguished with it.
Outcome Two: Victory for the Emperor
The foreseen outcome of the Heresy, the Cabal saw the Emperor giving his life to kill Horus, becoming a howling rictus trapped inside the Golden Throne. They believed that this path would lead to stagnation, and stagnation to decay. This decay would accelerate, continually allowing the Chaos powers to grow and spread their influence, until after ten or twenty thousand years of misery, the entire galaxy would be consumed by Chaos.
The Cabal, operating on the long view, sought to generate the first outcome by convincing Alpharius Omegon of the truth of their predictions. They asked him to place himself and his forces under the banner of Horus, despite their sworn loyalty to the Emperor, and do all they could to ensure the betrayer's victory, thus sacrificing humanity to destroy the Chaos Powers. As far as is known, the primarch of the Alpha Legion appears to have acceded to this request. As the Heresy ended with the second outcome, it appears that the Cabal's machinations failed and that, if they are to be believed, the galaxy is doomed. The Alpha Legion are still branded traitors to the Imperium and the Emperor, although their true motivations and loyalties since meeting the Cabal are not currently known.
Organisation and Abilities
The Cabal are a collection of disparate species that seem, in the main, not readily identifiable to Imperial authorities. They are known to include seemingly mechanical life-forms as well as those apparently energy-based. Others variously resemble insectoid, avian or lizard-like creatures. One species definitely identified as having a member in the Cabal is that of the Eldar. They are also known to have had at least one human member, although the individual in question appeared to act as more of a subordinate agent than an equal.
A nomadic organisation, the Cabal has traveled all over the galaxy. Occasionally choosing to spend a period of time in one particular place, they are known to have selected 'halting-sites' in particular systems, choosing worlds to set down on. These worlds had their topography altered to suit the needs of the Cabal, who appear to construct their landing-zones in an identical fashion. Worlds that they have set down on are therefore identifiable if one knows how to look at the terrain, two such worlds being Nurth and Eolith. As well as these geo-forming abilities, the Cabal also appear to have a high level of mastery over atmospheric alteration technology and the ability to extend or renew life. Their principal ability is an enhanced form of farseeing, referred to as the Acuity.