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?

Twilight breaks Celestia's doors.

But then, you'd be mad too, if your stupid brother got drunk and told you that you were adopted...

Princess Luna's Lunar Stallions kidnap Twilight Sparkle when they see how lonely Luna is, and give her to the Princess as a gift. Nightmare Moon emerges from Princess Luna's psyche during the sleepover. The dark goddess would love nothing more than to have the beautiful mare all to herself, and the resulting chaos forces Celestia to call a psychologist to sort out Luna's multiple personalities.

Twilight Sparkle wakes up one morning to a harsh dose of unreality. She is stranded, without her friends, on an island which matches no known locale on Equestria. Surrounded and harried on all sides by the alien and the dangerous, she will find her mettle sorely tested, and find perhaps that the best friends you will ever have in your life are the ones you make when your back is to the wall...

Thanks to CedricBale and noobie56xx for tracking down the name of the artist so I could give them credit.

We all wear a mask called composure, beneath which hides a heart that twists with guilt and hidden feelings. Regal as she is, Princess Celestia is no exception. What might be revealed should circumstances cause this mask to slip - or break?

Set several years after the potential conclusion to the series.

Twilight becomes alicorn of the stars. This is sort of a problem, because Luna kind of already was alicorn of the stars. Oops!

TV Tropes

For almost a thousand years, the immortal Nightmare Moon has reigned over Equestria as Empress. To the average pony, Nightmare Moon is an overarching enigma whose rule is absolute and whose wrath is terrible. But to Twilight Sparkle, who as an orphan has never known her parents, the Empress is something else entirely. When the young unicorn is brought under the royal wing as an apprentice, she learns a great deal more about the supposed tyrant than she ever knew. But as Twilight is introduced to life with her new mentor, she is thrust into a world of political intrigue, conspiracies and secrets. Secrets that were never meant to be revealed. As allies and enemies plot against the very peace of the nation, will Twilight fall prey to the perils of her new position? Or will she prosper and find in Nightmare Moon the mother she never had?

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My eternal gratitude to my editor, LordOfTheWrongs, without whom Night's Favoured Child would not be the story it is today.

Now with added TVtropes!

Her beauty, sullied; her wisdom, corrupt; my love for her, violated. But still I answer...

Summoned to the palace in the dead of night, Twilight takes up a burden that even her generous, loving soul may not be able to handle...

Humanized Twilight/Tyrant!Celestia.

The Elements of Harmony are defeated and Nightmare Moon stands on Equestria's throne as their new Queen. Is Equestria doomed to suffer through night time eternal or will they be... perfectly fine? Twilight finds her new job as slave or "personal assistant" to her new ruler to be a bit maddening. But is this evil sarcastic queen really as bad as she originally believed? And just how many others are diabolical enough to cash in on the change in staff? Can Nightmare truly run a peaceful Equestria? Let me just answer all these questions right here in the description...