Hm.....from the plot so far.....the Fallen have come to Earth after the collapse...also it is mentioned they are scavengers...yet all the 2 battles on earth referred in the campaign are against them.there is no mention of the events in the actual collapse.if they come after the collapse that means that they weren't in the Darknesses main army ....so either they are not minions of the Darkness and have come after his defeat or they are something like auxiliary units. The only reference to the Cabal it's that they are a warrior race and that their empire spans countless galaxy's.there is no mention if they do the bidding of The Darkness. So far the Vex are an machine race who master time travel, but with all their logic they cannot explain a powerful force they have encountered so they choose to worship it....is this force The Darkness? And the hive.......it's evil.....so they are in league with The Darkness. ....but their God its reffered to be Crota. .. So the missions don't explain much either...