"So many questions all at once!" -Kura
"Very well. We'll answer all of them." -Kuro
[Gonna summarize it all at once since it's late here and dialogue for these two is becoming a bitch.]
The duo explain that the Supreme Darkness is an entity that has existed for "as long as time itself" according to their master, though they don't know the specifics of how it came to be. The Darkness enters universe after universe, gradually devouring everything it touches, corrupting everything bit by bit. Those devored are warped into facets of the Darkness itself, reborn as puppets to do carry on its will, which seems to be nothing more than consume all of existence. It's become like a "hive-mind" of everything it ever consumed, bent on enveloping everything else that exists.
Your souls are intertwined with many more than the one you currently hold. The Emissary managed to rake a small few free of the Darkness' grasp and into an isolated universe [enough for one per player], but they're still corrupted. Your first mission is to recover these souls and return them to your own. This can only be done with those who were once your past selves [wishlist].
Once everything is all over, assuming things go according to plan, you will be able to return to your home universe with your powers intact, though if you prefer, The Emissary can remove them along with your memories (If he's still out there somewhere).
Their master disappeared abruptly, and recruiting you all was only a backup plan. He never would have wanted to involve "mortals" (for lack of a better word) such as yourselves had there been a choice. They believe that he fact that he has, along with his disappearance, means some unforseen circumstance occured.
[Insert any further questions/explanations here.]
K&K step aside from the door and gesture invitingly towards it.
"If you've resolved yourselves to join the mission, please come through this door." -Kura
"Alternatively, if you choose not to participate in this any further, you may wait here. The door on the opposite wall leads to the living quarters, where you will find your very own room. Feel free to rest there if you like." -Kuro