Luna stood in the back of the room while they argued over a plan. She noticed Stardust was missing, and decided that she had gone to find Twilight herself.

"I will be right back," she told the others. She left the room, then flew away.

Starburst crept out off her hiding place.

"Twilight! I know you are in there. Fight it," Stardust said.

"Foolish filly. Twilight is no more."

"You're wrong! I know she's still there. And I know she can hear me!" Starburst charged, her eyes glowing. She let go of the magic building on her horn. It raged towards Sombra, who dodged it. She charged up he magic again. This time it was a ball of chaotic sunlight, blazing with all the heat of the stars, and the cold of the moon. It hit Sombra dead on. Sombra emerged with frostbite over top of his sunburn. He attacked, black chains grabbing Stardust around the throat. Stardust couldn't breathe. Her eyes glowed bright as she overpowered the dark magic.

Luna burst through the window. She summoned her sword and stabbed Sombra. He grunted in pain.

"Luna! You know as well as I do that you can't beat me like that!" Sombra shouted.

"You may be right, but I will not go that far ever again."

Stardust tried to hit Sombra with her magic, but missed. She was wrapped in stronger chains.

"Let her go, Sombra. She is innocent."

"Never."

With this, Luna became outraged. Her eyes glowed, her form growing taller than Celestia's former height. Her coat became black and her mane flowed around her. Sharp teeth and dragon-like eyes completed the transformation. Luna was Nightmare Moon.

Author's Notes:

I made a joke. Rarity says the same thing. Sorry about weird video, best I could find without entire song.

https://m.youtube.com/watch?v=_QBtVdnzwog

Also, Nightmare Moon.