Ooooh boy.

What if the final origin character is this "Absolute weapon" that everyone keeps talking about in Act 1?

You know, the Beautiful Woman who crashed on the same nautiloid as us, and that everyone started following? It sounds like she has the ability to puppeteer people, and to always stay one step ahead of the party. Perfect for a hidden antagonist.

We assume that it is the mystery box that Shadowheart is carrying (and that the Gythianki are desperately looking for) but they actually seem to be two distinct elements. It's also NOT Wyll's Special Lady; she's always treated as a prisoner in dialogue.