Walkthrough:Kingdom Hearts Birth by Sleep/DarkestShadow/Pt 6

Space... The final fron- oh look a shooting star, lets chase after it Shadows, I, DarkestShadow, will lead you!

Vanitas
When we leave that world we see a cutscene of Roxas Ven flying through space on his Keyblade, then the Masked Boy show's up in the form of a shooting star like thing, so of course Ventus (and us...) will chase after it.

Head over to the new world, when we land in ??? we'll get a cutscene of Ven confronting the Masked Boy, who'll then bring out a Keyblade, then the battle starts