Written as an entry to the Swapped Roles contest (with the theme of Reunions).

For readers who may assume this is a generic "Luna and Celestia swap" I beg you to keep reading.

Princess Luna has been ruling Equestria for a thousand years, but as the banishment she enacted long ago comes to an end, she only has her student and a collection of random ponies to rely on, to save the world. If they can save anyone, it'll be just in the nick of time.