We've seen how they are physically stripped from Terra and Aqua, but I'm wondering why the armor and Ventus are separated in the first place. Aqua's magic sent her set into the RoL, and Terra's "mind" bound his armor back together. Ven's armor ends up following his heart into Sora's - is this a defense mechanism?
As for the possibility of Ven being a dreameater, I don't think we should categorize him as such just yet. Vanitas could imbue Unversed and had the symbol on his sentiment, but he was just the darkness (not negativity) extracted from Ven's heart, not an Unversed himself.
To stray a bit off-topic with you guys. If Eraqus did indeed take refuge in Terra's heart, then there would be a possibility of Xemnas having access to his sacred knowledge, that which he taught Aqua, thus allowing Xemnas to mimic a chamber.
That would only hold true though if the armor is that closely tied to the heart. The last time we saw Ven's "physical" armor was when Mickey found his comatose body drifting in the Lanes between holding hands with Aqua.
All subsequent scenes have Ven in his civil appearance.
We do see Ven's heart (I assume) donning his regular armor though lying apparently unconscious on Sora's awakening platform in that same trailer, so it may be actually possible that Ven somehow used the armor in order to try to defend his own heart and/or Sora's from something.
I also won't designate that blackened armor as Ventus himself, as we see him in his normal armor shortly afterwards. Yet that thing having the form of his armor already proves that it is related to him.
I already said what I think of that two times earlier so I won't repeat it a third time.
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Now that you say it, this would make sense if Xemnas had somehow access to Eraqus's memories.