It was almost dawn. I stared down at the meadow from my tower, my eyes burning with fury. The ponies were sleeping through the most beautiful meteor shower I had ever created, even though I'd made a speech telling everyone to stay up.
My sister trotted in, glowing with perfection. It was time for me to lower the moon so she could begin the day. Celestia approached, yawning.
"Luna, hurry up and lower the moon already," she snapped, glaring down at me.
I was shaking with fury. How dare they? How dare she?
"No," I said quietly.
"What?" Celestia demanded impatiently, rolling her eyes. "If you don't lower the moon in TEN SECONDS, Luna I swear…"
I didn't move, but something else shifted.
A strange feeling came over me. It was as though a new mind was rearing inside my own, taking control of my being. But it wasn't bad. I wanted her to take over, to be me.
Power coursed through my veins, staining my coat a deep onyx and lengthening my feathery wings until they were sharp and batlike, with skin pulled tight. My whole body grew taller, thinner, more angular, until I towered over my sister. I felt my mane change, as though it was not a liquid. Glancing at it, I saw it was dark blue and glittering.
A new being occupied my body, or rather, we shared it. The other being was vicious and insane and had probably been there all along, waiting to rear her powerful head.
"Luna-" Celestia began, but before she could finish, we reared and sent a fierce ball of energy at her. She crashed backwards into the stone of the castle, shock and fear clouding her face.
"No." we snarled, pointing our newly sharpened horn straight at her heart.
Celestia trembled. "Luna, let's talk about this. We can compro—"
"NO!" we shrieked, slashing at her with our horn. She screamed as we sliced through her flank, leaving a deep wound that slowly stained her white coat scarlet.
"Luna," she whimpered.
"Don't you EVER call us that again, you understand? We are not Luna! We are Night Mare Moon, and Luna is dead!"
Celestia had tears in her eyes. "Don't make me fight you."
We laughed derisively, insanely. "Fight me! You speak as though you have a chance!"
Celestia stood shakily, her cut dripping blood. "Please. Look inside yourself. Find the pony you truly are, little sister. Please."
We reared again, snarling viciously, and lightning crackled around us. Our newly starry mane flowed with the power that occupied our being completely, entirely.
Celestia was crying, but she raised her horn. She was shaking with misery.
A glow surrounded her, and when she opened her eyes, they were a burning, bright white. She sent a rainbow careening at our body. It wrapped around us, burning like fire as it enveloped our being.
No, MY being! A voice seemed to scream in our head. It was the voice of a filly, young and scared.
For one moment, one crucial moment, Luna controlled our body, forcing us down, protecting her sister. She allowed the power of the Elements of Harmony to seal us in the moon, a glittering shadow that would haunt the night for a thousand years to come.
Celestia stared up at us, now with tears coursing down her cheeks.
"I love you, Luna. Come back to me."