The hive do seem like zombie fallen and the darkness seems connected to them (at least in the beta) when you first fight them you get no respawn dark area so it makes me think that the darkness is the dark version of the traveler using dark instead of light and brings fallen dead back to life corrupted in to what we know as the hive (no freewill just a hive mind) The fallen are very intresting because we know that some align themselves with humans (the awoken anyway) at the reef [spoiler]possible 4th race in future dlc?[/spoiler] the hive however dont align with anyone (correct me if im wrong) and seem like the flood off halo