Only a few years after Princess Celestia takes the throne, her little sister finds an ancient library in the Everfree Forest. As she searches, she finds a secret room containing a black dusty old book. The book, once opened, teleports her to a dimension bathed in darkness, but yet resembling Ponyville. A shadowy being promises her respect from her kingdom, but there is a price. She learns that he is a parasite that infests beings and takes control of their body, turning them into a dark pony of nightmares. And there's only one way he's going to let the Alicorn get her wish. He wants her to bring him close enough to a specific pony so he can take them, to get a more powerful host. And it just so happens that the pony he wants is Princess Celestia herself.