Author: Pinkie Pie, shut up! No you can't name- You already did. Son of a...
Pinkie Pie: So author, you're always writing stories about me! How about I write a story about YOU!?
Author: That is a horrible... horrible... HORRIBLE ide-
Pinkie: Spectacularific! Let's do it!

A hack author (from "Extremely Terrible and Horrifying Stories" and "Mystery Pinkie Pie Theater 3000") is attempting to write his next magnum opus. Then Pinkie Pie happens.
Not the most original idea in existence, but hey, this is my take on how it would play out.

On a dark night filled with even darker magics, an insane cult attempts to give Nightmare Moon a body and life of her own, utterly separate from Princess Luna. But, when the spell is interrupted by Celestia, something unexpected is created.

Now, a little black alicorn filly named Nyx finds herself living under the care of Twilight Sparkle, only to be haunted by memories and emotions from her past. Is she Nightmare Moon reborn or is she simply a doppelganger with a soul and mind of her own? Can Twilight Sparkle protect Nyx from those who refuse to see past her dragon eyes and black coat?

Or, will Nyx be forced to inherit the sins that may not even be hers and become the greatest evil Equestria has ever known?