Maybe when Ansem and Xemnas Were defeated, Master Xehanort, Terra, and Eraqus all reformed as seperate people instead of as Terranort?
Or at least Terra and Master Xehanort. The question is, if they reformed seperately, where's Terra? If he is one of the Darknesses, then he reformed before Xehanort. Otherwise, there would have been two forming people in thrones, not one.
The most confusing part though, is why Xehanort appeared in Where Nothing Gathers, and not at Radiant Garden. And if Terranort went there before splitting himself, Terra should be there too.
I have one theory on this, though. I think that they are in the Realm of Sleep during that scene. Considering Riku is younger as well as Sora, I believe the 11 other vessels (besides MX and Sora) went into the Realm of Sleep, then lured Sora to a second layer of sleep and took his body to the sleeping CTNW, where they waited for Xehanort to form in the real world and then go to sleep. I think what we are seeing is Xehanort appearing in the Realm of Sleep, by going to sleep, not being recreated there. Xehanort actually reformed in the real-world CTNW, where the other vessels are also asleep. Basically, imagine all the Darknesses sleeping in WNG, and all we see in the game are their dreams. Terra would have also reformed in the real WNG, but before Xehanort, and when he awoke, the other Darknesses pounced on him and put another Xehanort heart in him. That is, of course, if they reformed seperately.
When the scene is over, the Darknesses all "dissolve", teleporting away. From what I gather, most people think that they have returned to their times, but why would MX, Braig, Isa, and possible some of the other Darknesses do the same? They exist in the present. I believe that "dissolving" is what it looks like when you leave the Realm of Sleep. Basically, the XII Darknesses woke up, and are probably sitting comfy in the real Where Nothing Gathers, plotting their next move. And for those of you who will quote what YX said about returning to his own time, remember, he used the word "soon". Much like Nomura's "soon", this could be in a year.
For some reason, I think the only way for them to return to their own time, is after they fulfill their purpose and forge the χ-blade. They they return to the exact moment they left, their memories of other times wiped, then proceed to get their butts handed to them by Sora. XD
So, to recap:
-YX gathers the vessels of MX and brings them to WNG.
-Terra reforms and is forced to be a Darkness.
-11 Darknesses go to sleep. -Xehanort reforms in WNG, then goes to sleep to greet his final vessel.
-They fail to acquire Sora.
-They wake up.
-They send YX to go find a new vessel(?)= beginning of KH3
-end of KH3= χ-blade reforged, Xehanort has no more need of vessels, so he sends them back to their own times. Braig continues to help him(?), Isa joins the good side, Terra as well, then final fight begins.
Well, it makes sense to me. :/