"There can only be one princess in Equestria! And that princess... will be me!"
Luna's soul is screaming.
In the first few moments, it'd felt good. It'd felt so very good to lift the moon into the sky and shroud the sun. The concept of an eclipse had already existed, as previous keepers of the heavens had experimented with such things, but there'd been no living memory of them, the word having come to mean so many other things in Equestrian, with its original meaning lost.
Including what Celestia had done. She'd eclipsed Luna, leaving her younger sister wasting away in her shadow. The Avatar of the Moon had been lost in the brilliance of the Avatar of the Sun, and so it only felt fitting to blot out the sun with the moon, and reverse the dynamic, if only for a little while.
It'd only been meant for a day. Luna remembers that now. She'd only meant to keep the moon in one place for a single day's length, then lower it again to return to the proper cycle. Just long enough for everypony to see how beautiful things looked under moonlight...
Then she'd been before her sister, on the cusp of it, and the words that spilled from her lips...
She can't see Celestia anywhere. The magic surrounding her has obscured her, and more magic is cold fire rushing through her veins. Everything is stretching, twisting, contorting, and she swears that she can feel bones grinding together as parts of her reshape.
Her body is stationary as the changes take place, but Luna is still screaming. Her mouth is simply no longer her own to let the sound out.
There are faces in the whirl of shadows, and Luna doesn't know if they're real, prophetic, or hallucinations of a shattering mind. She sees her Court of Dreams, each and every nyx twisting and warping and howling in pain.
Spectacled eyes are staring at her, gaze silently asking why?
Doran
My first friend
I'm sorry
There's laughter. There's laughter and it's not hers. It's a twisted, cruel thing, a parody of true laughter and it's coming from her throat.
"Luna, I will not fight you! You must lower the moon! It is your duty!"
For years she'd steadily found her teeth more on edge every time her sister spoke, reading sarcasm and condescension into every kind word and attempt to reach out a hoof. She was convinced that Celestia pitied her at most, rolling her eyes secretly at her descent into misery.
Now all that is gone. This thing has gluttonously devoured every bit of resentment, hate, and rage, and grown strong on her despair.
I let it in
My heart let it in to feast
I failed
"Luna? I am... Nightmare Moon! I have but one royal duty now: to destroy you!"
All she has left is horror as she beats her hooves on an internal cage. She's being dragged down, the voices distorting as if she's being submerged into icy waters.
She has time for one last scream before she drowns.
CELESTIA RUN