Great story. Here's the summary the author gives, something far better than I could do:"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, an 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 the filly Nightmare Moon reborn or is she simply a doppelganger with a soul and mind of her own? Can Twilight Sparkle protect Nyx from those 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?"Here's the link to the story if you're interested.