Deep in the Everfree Forest lies the castle of the ancient pony sisters. Princess Celestia lived there before Princess Luna became Nightmare Moon and destroyed the castle. After Celestia banished Nightmare Moon to the moon, remnants of Nightmare Moon's power remained. Every night, these remnants come together and form the Shadow Pony.
Luna hears a knock on the door outside the Star Swirl the Bearded archive. She uses her magic to unlock the door and Twilight Sparkle steps in.
"Hello, Princess Luna! You said you needed me?" says Twilight.
"Yes. I need help finding a book," replies Princess Luna. Twilight gasps and looks around the room. Books and papers are strewn across the floor and shoved in every nook and cranny. "It's a dark blue book, and it has a moon on the cover. I don't exactly remember the name of the book, though," explains Princess Luna.
"I'm on it!" replies Twilight Sparkle. She dashes around the room yanking books off of shelves until she finds the correct book. "Found it!" cries Twilight Sparkle, "There's no title on the cover, though."
Luna magics the book towards her and stares at the cover. "There's a title right here, Twilight," says Princess Luna. She points her hoof to a blank area on the cover.* Twilight trots over and peeks at it.
"There's no title there, Princess Luna. Are you okay?" says Twilight. Luna glances at her quizzically.
"The question is, are you all right, Twilight Sparkle? The title says Coho Sak Tu Moone. It's in one of the languages that Star Swirl the Bearded made up. I hope that the rest of the book is not like this, since I have no idea how to translate it," says Princess Luna. She flips through the pages. Little does Luna know that the title is actually a spell to release the Shadow Pony.
Back at the castle of the ancient pony sisters, the shadow pony is hit with a beam of darkness. After all of the smoke clears, the shadow pony stands up and looks around. It realizes that it is not a shadow any longer and magics a mirror over to it and looks. What it sees is Nightmare Moon.
* The title of the cover is only visible to Luna herself.