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..
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It seems there's been some misunderstanding. I'm not saying I believe this is canon, it's just a wild idea I had that I mused about and thought it was interesting. With that said, I'm copying and pasting a list I had created in response to someone, because I want this list to have its own comment rather than be lost in a shuffle of replies. A list that would support this headcanon of mine if it's not approached with dismissive attitudes but rather approached with the same playful attitude I had when I came up with it :) - The physical similarities between Mara Sov and Eris (srsly, google them and place them side by side) - The time collapse of the Vault of Glass with the Taken infestation - The cutscene of the guardian using the stealth drive to travel to the Dreadnaught for the first time showed floating corpses of the Awoken that fell in battle, yet Mara Sov's body was nowhere to be found despite the efforts of Petra and the other Awoken - Mara Sov had extensive knowledge of the Black Garden and the Vex gods, even playing a huge role in showing the guardian how to open the garden, and its been confirmed that those Oryx takes, he gains the knowledge that they possessed too - Eris Morn led a team into the place where Crota lay - Eris Morn was the only survivor - Eris Morn doesn't currently possess the Traveler's Light - It is claimed that Eris had her light "taken" from her, it would have to be an extremely powerful minion of darkness to be able to do that, yet she miraculously survived an encounter with such a being even though her light did not - When the opening TTK cutscene ends, Mara Sov says The Awoken have played their part. If this theory is to be believed, then that quote gains a deeper depth - When Oryx retreated, he did so with a smirk, it is believed in arrogance, but what if it was because everything was going according to some plan and then Eris took the core of the Sword