When Starlight Glimmer steals a time travelling spell, threatening the fate of Equestria, Luna decides it's time to find out why Celestia hasn't been doing anything to stop it.
A tiny filly stands on an open plain. Somewhere in the distance a fleeting glimpse of white leads her on, her heart yearning towards it. A dark power with her face attempts to stop her. And nothing here is quite what it seems...
Long ago, in the frozen wastelands of what would become the Crystal Empire, a lone unicorn stallion becomes trapped in an icy tomb. He finds salvation, but not without a price. A King Sombra origin story.
She looked just like Princess Celestia, except marred and broken. Her ceremonial barding had rusted slightly. Black streaks ran through her mane. Worst was the scar across her chest. The tarnished Celestia smiled at Luna. "H-hello, monster."
Princess Nightmare Moon had tried to keep it secret. She had feigned ignorance, but the charade could not last. She nocked the arrow. Tirek was a large target; he would fall easily. Then the true challenge would come: explaining herself to Celestia.
Nightmare Moon won. She defeated Celestia, broke the Elements, banished the sun. This all happened a very long time ago. So long ago, that she has had plenty of time to change her mind. Not that fixing it will be simple.
When nature ran wild and free, ponies had only begun to dream, and a different moon roamed these skies. So it might have stayed, had the deer in their pride not turned to Wrath and to Ruin. That time is gone, but its legacy—and its lessons—remain.