[Sora frowns, catching on at least enough to realize this is all probably terrible news for her.

After a moment he moves closer and gently puts a hand on her arm.]

Yeah...

[He says it more carefully this time.]

I don't know all of the details, but the first one was pretty bad. It... broke the world into different pieces and darkness won. The x blade shattered and Kingdom Hearts was lost.

From that, the worlds as they are today formed. And now each one is separate and different, with borders that prevent normal travel between them.