I want to start off by saying that this is all base off of Grimoire and the in game lore. [b][i]The Vex[/i][/b] Who created them? Why are they here? Why are they attacking us? These are some of the most common questions asked of the Vex. I would best be able to describe the reason they are here, as backup. In the future, something, possibly the Hive, caused a sort of apocalypse and the Vex were sent back by the Traveler to aid in our (inevitably failing) battle. So why are they attacking us? I believe they are attacking because when the Vex discovered the Black Heart, they defected. The Black Heart was much more powerful than the Traveler, it in a sense, recreated the Vex, into celestial gods, rather than mere soldiers. The Heart made them into pawns that would have the power to create a universe (not unlike Crota). They created the Vault of Glass as a testing ground, the Vex are gods in the Vault. Reality bends to their will, and if they are not stopped soon, their power would have no boundaries. [b][i]The Fallen[/i][/b] The Fallen are just that, fallen. The remaining Fallen houses are just remnants of a fallen empire. I believe that the Fallen were once soldiers of the light, and that they too, served the traveler. Unfortunately, the Traveler eventually would come to realize that the Fallen were cruel and unjust. The Traveler would then leave, causing an overbalanced government, so the Fallen split into five houses. These houses would end up trying to recreate their lost empire, in that particular house's image. This is why the Fallen still hunt the Traveler. [b][i]The Cabal[/i][/b] These 800 pound monstrosities are well known to destroy planets, just for being in their way. The big question is, why haven't they destroyed us yet? I believe that they are running from something, and that they may have encountered a sliver of that horror somewhere in our star system. So then why don't they just leave? Well perhaps they are searching for something on Mars, something valuable enough to risk extinction. [b][i]The Hive[/i][/b] Gods, zombies, an infection? What do we call these horrifying demons? I believe that the sole purpose of the Hive, is to plunge the universe into a state of complete, utter darkness. I also believe that the purpose of the Hive very strongly ties in with all the other enemy species. The Cabal are running from a prince of the darkness, maybe its king, Oryx. All we know, is that if Crota hadn't been on the moon, they would have long since destroyed us. Their relation with the Fallen could be as simple, as that the Fallen were one soldiers of the light, but they fell to the might of Oryx. The Vex seem to have the deepest connection with the Hive. The Vex were sent back to aid in the fight against Crota (that is, until they discovered the Black Heart). Although, Grimoire subtly implies that the Black Heart could have been one of Crota's siblings, if not a part of Oryx's soul. Thanks for reading! Be sure to like and comment on my mistakes or changes you would have made! Edit: Thanks for all the support!