So here’s the scene that really brings it all together. Xehanort, shortly after Vanitas is formed, makes the connection between him and the ancient darkness. He knows about what happened to Ven from his dreams and is almost certainly aware of the prophecy (one of the deleted lines from khdr even references a ‘chosen one’ who harbors darkness similarly to khux’s prophecy).
So that’s a lot of new information from KHUX and KHDR—but does any of it *actually* contradict BBS? Just what is Vanitas? Darkness? Or part of Ven’s heart?
Well, hopefully you are following my train of thought here, but it is pretty obvious to me that this whole argument is really stupid because the answer is just ‘both’. Darkness needed Ven to exist as a being with will, in that sense, you could say it originated from Ven—or you could say it existed before him but was shaped by him into something more willful and individual. Ventus is made pure-hearted in the process of dividing. Darkness seeks to possess human bodies with hearts and Keyblades so it can lay claim to more worlds, and Ventus is the one chosen for that role. Ventus vows to give Darkness a new “form” that can be defeated, and traps Darkness in his heart, meaning that it can be defeated if he dies. He however does not die and ends up in the bbs era. Xehanort attempts to extract Darkness from his heart, correctly knowing that Ven is harboring it, but takes a fraction of Ventus’ human heart in the process, which Darkness forms around, and since he has a form and a heart, he is able to wield a Keyblade as Vanitas. Vanitas is distinct from Darkness because of Ventus and distinct from Ventus because of Darkness. Ventus and Vanitas rejoin at the end of Birth by Sleep, and Ventus wins back the piece of his heart that was in Vanitas, and the Darkness also returns to his heart (as Sora sees in Re:mind), meaning he is in the same state as he was before the split took place.
If you have followed my logic so far, you get a gold star sticker. ⭐️
In my humble opinion, absolutely none of this changes Birth by Sleep. Vanitas’ motive is to be in control of Ventus’ form, to spread Unversed throughout the worlds, and claim Kingdom Hearts. These are exactly the same motives as the Darknesses in KHUX. Ven’s story themes of sacrifice to save his friends over himself are present in both UX and BbS and the two stories heavily mirror each other. Both stories still feature Ventus confronting his dark side / overcoming dark desires. Xehanort always used wording to indicate he believed that Vanitas was inhabiting Ven’s heart and he just took him out of it. Ven still won back the piece of his heart that was the driving force of the bbs plot and trapped Darkness which was the driving force of the khux plot. I think the fact that Vanitas was briefly distinct from the other Darknesses because of his form / heart will be an important plot point going forward, but I’m not going to speculate about future games in this post.