Light of all colors shot from the stone orbs that surrounded Princess Celestia. The light spiralled to create one large beam which shot her opponent with a deluge of prismatic light.
The dark winged unicorn underneath the light cried out in agony. Her purple plate mail broke off in large chunks and melted away. Behind the winged unicorn, a cone of shadow faded away as the stream of bright light took its place.
Princess Celestia’s glare soured. Even the pony underneath the armor did not sound like her sister. Nightmare Moon had already poisoned her mind. She thrust her head forward, causing the hovering orbs to cease the flow of light coming from the symbols inscribed in magic stone.
Nightmare Moon continued to stand. Behind her, shards of thick armor steamed with a bright aura. Her legs suddenly lost balance, but she forced herself to move forward. Her once proud, sky-shattering step turned into a malformed hobble. She tried to speak, but white smoke escaped her lips. Her voice, once imbued with the timbre of the night, had been stolen by Princess Celestia’s light.
Princess Celestia readied her horn for another spell. The six orbs shot up and glowed with light. Perhaps another spell would return the light to Nightmare Moon’s eyes.
”That’s quite enough, Luna.” another voice said, “I thought we agreed that we would switch places once you were done.”
Nightmare Moon’s eyes widened. She tried to speak, but her words came out as voiceless smoke.
The lumps of purple armor slowly rose into the air. They swirled in the air, twisting and melting into a sphere. The orb released a flash of blinding darkness.
A strange creature stood in between Princess Luna and Nightmare Moon. It was a slender serpent with mismatched legs and arms from different animals. Horns topped the serpent’s head, but they were also mismatched. It floated in the air in spite of its immobile wings.
Nightmare Moon looked at Princess Celestia helplessly. It continued to mouth words, but Princess Celestia heard nothing.
“You know the deal. When you’re ‘done with your sister,’ I take my payment accordingly. You did so well that you were ‘done in by your sister.’” The creature clasped its mismatched arms. “Like we said, I’m here to take my payment. You know what happens when you’ve squandered my time.”
The serpent turned its head towards Princess Celestia. It smiled at Princess Celestia, then winked.
“I’m sorry, summoned-to-summoner discussion.” the serpent said dismissively, “we’ll be done soon.”
“Now,” the serpent turned back to Nightmare Moon. “It’s time to become Nightmare Moon, Princess Luna, because it will be some nightmare when you are the Moon for a good millennium.”
Princess Celestia threw down her horn. Light shot from the orbs and gathered into one beam. The light charged towards the serpent, but sped past the creature like a large stone in between a stream.
The serpent raised one of its mismatched arms and thrust it downwards. A burst of darkness enveloped Nightmare Moon. When the darkness lifted, Princess Celestia saw nothing but the stone floor of the castle.
“Where is my sister?” Princess Celestia growled, “what have you done to Luna?”
The serpent pointed towards the moon.
“Up there. But while she’s up there, I’m her replacement.”
The serpent cackled loudly. Princess Celestia cast another spell at the serpent, but the light from the orbs averted the slender creature.
“If you’re the light, then I’m the dark. If you’re happiness, then I’m sadness. If you’re life, then I’m death.”
Another spell shot towards the serpent. The feeble spell bent away.
“If you’re Law, then I’m Discord.”
An interesting alternate history here, where instead of Discord being sealed by Celestia and Luna before Luna’s banishment, Discord IS the Nightmare – and the one that ends up banishing Luna. Discord establishing himself as the yang to every one of Celestia’s yins was also intriguing.