I believe the cabal once had a vast empire that had conquered many worlds. Then they found the darkness...and advance parties were sent out to the stars to save the empire, find a way to fight back. The cabal found our system and the traveler and trenched in on Mars. It is possible the cabal homeworld was destroyed by the darkness but it is known the cabal on Mars have lost contact with the emperor, and are divided about their purpose and methods to achieve their mission, with infighting among some of the Val and valus. Since the cabal are militaristic, I believe them to be much more interested in the warmind than the traveler. I mean, the traveler is damaged and unresponsive but Rasputin is still alive and raining fire from the sky. All the grimoire and story missions talk of the cabal searching for the warmind on Mars, almost indifferent to the traveler. Cabal want a weapon to fight the enemy that they don't understand (darkness), and that weapon is Rasputin.