Created in an attempt to resemble an actual episode of the show.

Luna has fallen into a deep coma from which she cannot be awoken no matter how many people come to her aid or whatever spells and medications are used. And her sister's knowledge of the dreamscape isn't as profound, preventing her from creating any real solutions. Celestia did, however, discover, in her brief trips into Luna's dreamscape, the words 'Promise' and 'final wish' floating within. She does not know what they mean.

Forced to stand by her sister's side at all times, Celestia sends the mane six, being the only ones she feels she can trust in this delicate situation, towards a potential solution which lies deep below Canterlot in a newly discovered area that has yet to be explored, much to Twilight's certain excitement. While it isn't certain that a solution lay within, there is no harm in trying.

Takes place, perhaps, at or after the finale of season 5. Non-canonballically, of course. It's also a makeshift birthday present for Lunaexcelsior. His birthday was June 1st, if I'm not mistaken. Joyeux anniverschtroumpf!