Like the title says, this is almost pure speculation but it blew my mind when I realised the possibility.
Important notes; Savathun has possesion of a taken vex mind 'Quria, Blade Transform' gifted to her by Oryx but the Taken King left it some will of its own. This combined with Ashers comments on Io about how the vex conversion process may give them some force of will over their taken aspect implies that Quria may intend to betray Savathun.
Here's the wild bit. There is a member of Oryx's court from TTK who's origins are shrouded in mystery, as opposed to most of the court who have their origins spelled out in their grimoire card. The card for Lokaar reads. [quote]Who is this nameless thing?
She fell upon Omnigul, whose aspects multiply, saying
I am diminished. Gift me your death.
My son Crota came to me, saying
This nameless thing moves sideways
I chased her, and she fled
Kill her now
So I raised her to my court
I named her Lokaar, which means Not There
Thus my son was rebuked against simplicity [/quote]
Lokaar, despite having amassed enough power to challenge Omnigul, was completely unkown to Oryx. And she was "diminished" so we know at a previous time she had even more power at her disposal. She seeks to kill Omnigul to gain power as is the hive way but the act seems so rash, this is a creature striking out in desperation. Lokaar also posesses the ability to move "sideways" as the hive put it, which in game is represented by teleportation, and used this ability to evade Crota, no small feat considering his raw combat prowess.
[b][i]Spinfoil hat time![/i][/b] Lokaar is Savathun. After being defeated, either by us or Quria directly, the vex mind is for some reason unable to kill Savathun. So to stop her reclaiming her power it sends her into the past, to a time where Oryx is still king and another Savathun rules the High Coven. Savathun in desperation attacks omnigul, ordering the wizard to "gift her death". Oryx then raises Savathun now Lokaar to his court and there she stays, plotting her resurgence. However before her plans can bear fruit a ramshackle team of guardians board the dreadnaught, summon her forth from the court and show her what for. A pathetic end to one of the potentially most dangerous threats we will ever face.
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<While I do believe Quria intends to betray Savathun, I believe the latter IS leading the charge for now. Xivu Arath is somewhere else with scouts in Sol because of its apparent constant war.>