User blog comment:B1littlehero/Speculation of the Origin of Species/@comment-160529-20140612050226/@comment-7622429-20140612121111

Well, I'm with ya there. The Cabal seem to be a relatively recent addition to the galactic Human/Awoken/Exo-hating party in comparison to the likes of the Hive, Fallen, and even the Vex. As for why they'd come here at all? Well, the Darkness is tied to all these races in some way, no matter how small. And you mentioned mercenaries. Remember when I called them "guns for hire" below? That's the sort of thing I meant. Perhaps the Darkness "hired" them to aid their quest to eliminate Humanity, and scour the ruins of their civilization's presence on Mars. Or maybe they just like Mars a lot. The Cabal aren't exclusive there, I don't think.

As for Psions, well, they're a lot smaller than normal Cabal, and have powerful psychic abilities, which I'm thinking is compensation for their small size in comparison to their brethren. Perhaps a natural and/or engineered augmentation for any member whom displays such scrawniness. Again, take my speculation with a grain of salt.