The Elements of Harmony didn't kill Nightmare Moon. They just peeled her away from Princess Luna. Small shards were all that were left. From those shards along with the remaining magic from the Elements of Harmony created Nightmare her own body. The bad in her was what was destroyed.

Now, she lives in Canterlot as a unicorn student under Princess Celestia. She has things going for her. She has friends, a steady relationship with a fellow unicorn, and nobody knows that she was Nightmare. They know her as the kind and loving Moonstreak.

She to was sent to Ponyville help Twilight Sparkle with her recent studies on another world. But when some things go wrong, the only way to save Equestria is to come clean. She must live with her past sins. Someponies see past her dragon eyes, but not others. She must put trust in fate.