Something is not right. How am I an evil, cold-blooded ruler here in this alternate world. They call me the "Princess of Darkness", when I am the "Princess of Friendship". It remains a mystery.

When all goes wrong during the "Sibling Supreme" battle, Twilight and her friends find themselves trapped under Celestia's Castle, held hostage by someone so close that they know the littlest of secrets. How will the Mane Six survive this ordeal?