The Cabal drill on Mars uncovers an ancient Vex Forge which they unwittingly reactivate. The Forge begins spawning a flood of new Vex, but these Vex are not like others encountered. The new Vex are other races that have been fed to the machine and reprocessed into new bio-mechanical lifeforms. Essentially keeping with the "Taken" theme, but instead of races getting "Hived", they get "Vexed". The Raid is assaulting the Forge itself and fighting Vexed Guardians. or The Cabal discover an ancient Ahamkara, a wish granting dragon. Through their innate pride, and their shame at having to ask Guardians for help with Oryx, they make a pact to gain the power of the gods. As we know (or anyone that reads the lore knows), the Ahamkara wishes come at a high price, and the new Cabal gods find thier wills being subtly manipulated by the Ahamkara. The Raid is finding and bringing an end to (what we assume) is the last Ahamkara.