I think we do fight a physical embodiment of the darkness in the future or we might get introduced to a completely new race, who are worshipping and have more relationship with the darkness. their connection with the darkness might be higher than hive or vex. how am i so sure?. Look at the latest voice lines of eris and drifter. the dialogues might not be accurate Eris - the hive would be growing larger and more powerful because of the darkness. drifter- Look 3 eyes, I have battle the dark, the hive are NOT the dark. eris- the hive are children of the darkness. this interaction is important as this actually hints a new enemy race coming to destiny which is the darkness or have a much stronger connection to dark than hive or the vex.