This story is a sequel to Saving Sunset Shimmer
Faced with Nightmare Moon's approaching return, Twilight, Tempest, and their classmates need help from an expert on the subject... a pony named Moondancer. There's just one problem: it's the last thing Moondancer wants to talk about...
For more of this AU, see: Trixie Luna Moon
And also check out the series, beginning with Not Exactly Friends.