For some reason I think Eris Morn is Mara Sov. I don't know how, or why. But Mara Sov's body was never found, and she said something about the Awoken playing their part in a story. When the assault took place, Oryx said he would take them all. What if HE took Mara's body and infected it, and then sent her to the past using a taken Vex in the Vault of Glass (where there was a time collapse, as seen in one of the missions featuring the voice of Praedyth). And in the past, she roamed around, eventually foreseeing the awakening of Crota and leading a fireteam of guardians down and feeding her essence to Crota allowing her to take the visage of a Hive - peculiar how she was the only survivor.
This would also make her taking the essence of the sword at the end of TTK make sense. It seems crazy, and it probably is, but if I look at images of Eris and Mara Sov side by side, I see a very eerie similarity between their appearance..