Looks pretty cool, could do with a bit of tweaking however. One: The Cabal have not expanded into the system past mars, the vex have been stopping them. If the Cabal moved further into the system they would be unstoppable, they would probably destroy the moon to get rid of the hive and the fallen would not be enough to keep them away from earth, the last city would fall in days if the cabal mounted an assault. Two: The vex would never accept a human into their ranks, however dark their intentions. "Evil so dark it despises other evil." Maybe they could instead have captured a dead guardian/ghost and re-animated/reprogrammed them using vex technology, we have already somewhat seen this with the fall of Kabr. This undead guardian's ability to wield the light against other guardians could make them an exceptionally dangerous foe. Three: There needs to be another enemy rather than the darkness, as I doubt we will confront the main enemy until at least the second game. Going back on my previous statement, perhaps it could be the cabal invasion that sparks this need to look for help. Just some friendly tips to help refine the idea. It's still very good and I'd buy it if it came out.