Unfortunately much of the game’s lore is hidden within the Grimoire cards.
Thanks to Reddit user KentsFood, he’s compiled information & lore about the Cabal (mostly from the Grimoire), which you can read below.
Brutally efficient and unrelenting, the Cabal are the greatest known military force in the system. Their origin is as much a mystery as their end game, but it is said they have conquered more planets than the Guardians have charted, and they are preparing for annihilation.
Cabal Detachments: Sand Eaters, Dust Giants, Siege Dancers, Bone Crushers, Sky Burners, Ice Reapers.
The Fallen are a broken race of scavengers, the Vex are unfathomable, and the Hive are pure darkness. The Cabal are none of these things. They are a sophisticated military culture, armed, armored, and organized for conquest.
Cabal tactics focus on Legionnaires advancing whilst being supported by the various other ranks and roles of the Cabal military. Legionnaires are the first line of offence and defence, and will either close the gap to the target or find more advantageous positions by bounding across the battlefield with their jump packs.
Some Legionnaires carry massive shields, these units are called Phalanxes. They use these indestructible shields to great offensive effect (as well as obvious defence), slowly advancing whilst hiding behind their shield. Guardians have reported that Phalanxes will lose balance if their firing arm is pierced.
Heavy infantry consist of the Colossi, huge Cabal wearing heavy armour and wielding deadly heavy weaponry. They are positioned where the Cabal need to push back an enemy offensive or push forward their own, and to great effect. More than one Guardian has fallen to the devastating hail of a Colossus Heavy Slug Thrower.
Commanding them all from the field are the Centurions. Centurions are responsible for relaying orders and replying munitions for the Cabal military effort.
Then there are the Psions. Psions are remarkably smaller than their Cabal counterparts, and are speculated to be of a different species all together. They are named after their unique psychic abilities, notably their ability to manipulate energy at will and possess computational equipment with their minds. It is believed they may have telekinetic abilities as well.
The Cabal are seen to utilise small scout vehicles called Interceptors for advancements. Whilst merely scouting devices to the Cabal, to Guardians they prove an effective tank. Guardians should capture these devices, where possible, for use in the Crucible for training purposes.
Goliath Tanks are formidable tanks that the Cabal deploy for effective area denial. Their weapons systems are surely capable of attacking orbiting space-craft. Do not underestimate their agility, the boosters allow for quick terrain traversal, but subsequently act as a weakness.
The Sand Eaters are the bulk of the Cabal presence on Mars. They are apparently equipped for a long deployment on the surface, but continue to pursue their unknown objective with trademark Cabal determination.
If the Sand Eaters are unable to hold a line, or they require reinforcements for whatever reason, then the Dust Giants are sent. The Giants are the elite Cabal units who ascended the ranks of the Eaters. They are used as shock forces and reserve troops and will be continuously sent in until the threat is neutralised or there is no alternative but to retreat.
If the Giants are the Elite, then there is no word to describe the Siege Dancers but extraordinary. The Dancers are deployed wherever the Cabal need a perimeter as quickly as possible. Their missions involve invading unknown territory and destroying key enemy fortifications.
After a Guardian took the Dust Palace from the Cabal, the Siege Dancers were the ones to take it straight back.
The Cabal are locked in a war against the Vex, a war they are winning due to the efforts of the Blind Legion. The Blind Legions primary goal is to delve into Vex structures and secure Vex artifacts, as well as destroy any of the robots they encounter.
Bracus Tho’ourg has been reported by Guardians since the Cabal were first encountered. Those Guardians are long lost, by records (now legends by all accounts) speak of Bracus seizing the Buried City with a fraction of the Cabal’s current force on Mars. This was during the collapse, showing just how long the Cabal live. Bracus was killed by a Guardian on a recon mission on the Cabal’s knowledge of the Black Garden.
Bracus Tha’aurn was believed to be the commander of Psions in the Cabal military, as he is only ever accompanied by them. Tha’aurn showed interest in the remnants of humanities Golden Age, and was found in the Buried City at the site of Rasputin. He was killed there by Guardians.
Primus Sha’aull was the commander of the Blind Legion. He was often reported risking his life fighting alongside his legion against the war machines. Sha’aull was killed by a single Guardian who bypassed the Legions lines and crossed Sha’aull off (much to Cayde-6’s amazement).
Valus Ta’aurc was commander of the Siege Dancers, and claimed more ground on Mars than any other known Cabal. Valus traversed the battlefields on his massive Tank. accompanied by his Lieutenants. Ta’aurc and his Lieutenants were killed in a strike by Guardians, assassinating Valus in his own Imperial Land Tank.
Finally, the three Psion Flayers were destroyed by a team of select Guardians. The extent of the Psions abilities was unknown, but they were able to take back the Dust Palace in a matter of days and infiltrate the computer systems there. Warlocks believe that the Psion Flayers are responsible for the removal of Phobos, Mars’ second moon, from Mars’ orbit, and that they are holding it until it can be used as a weapon.
A record of a conversation between members of a Titan unit on Mars some time ago indicate Cabal presence on Mars. The Titans were looking for the Warmind and “the one who guards it” in the Dust Palace, and mention how Ikora Rey (before she became a leader of the Vanguard) had a vision, presumably to do with the Cabal.
It is my belief that Ikora foresaw the Cabal invasion of the Dust Palace and tried to warn the vanguard of it’s threat. It always has been difficult to convince Titans to retreat.
Records exist from a Ghost, assumed to be the same ghost as mentioned in the above record, who tried to find a Guardian in the deserts of Mars but ultimately failed. The Ghosts did, however, find a way into a Cabal instalment and gained valuable intel on their hierarchy. The Ghost mentions a few things of interest. It speaks a vast empire beyond our system. It is assumed that contact with this empire is long gone, as some cabal dedicate themselves to it, but others do not. Perhaps the Cabal we are facing are insurgents, or a scouting force for a larger armada?
The Ghost also mentions a device that appeared to alter a planet, speculating it could be their home. This is unlikely, seeing as it appears all contact with their home as been lost. It then goes on to speculate about the purpose of the Cabal, ultimately coming to the conclusion that the Cabal, for all their might, are running. Running from the Darkness, from their own people, from something else. If something has the Cabal – a species so heavily militarised they have minimal security for lack of fear of invasion – running, we should fear it too.