Everypony grew up hearing the ancient myth of a city in the clouds that is the prison of a powerful Demoness. Celestia and Luna have kept much of the myth a secret. When the signs of Tambelon rise again, Celestia leaves Luna in charge so she can go ring the bell of Liberty and send Tambelon back into the realm of darkness. There is one problem. The bell is shattered. Tambelon ain't going nowhere.

Twilight Sparkle uncovers a secret hidden below the library that tells about the seven Guardians of Magic and how one Guardian subcumed to the darkness in her soul and was sealed away. She learns about Tambelon, where Celestia and Luna came from, and makes yet another shocking discovery.