To save her sister and her faithful student, Celestia herself must travel into the ancient tunnels beneath the ruined castle in the Everfree Forest. She, and the guards who accompany her, must face the dark, twisting corridors. Still, what was Luna and Twilight's purpose at the old castle? Why were they unable to return of their own accord?

And what watches Celestia from beyond the light's edge?

There is a secret in Equestria. A secret the newly crowned Princess Twilight Sparkle doesn’t dare to believe. Like puppeteers pulling on marionettes, they have influenced Equestria for better and worse since the fall of Discord. So far they have been benevolent, but their potential for evil is vast. What’s worse, she doesn’t know their name. She doesn’t know their mission. All she knows for certain is one thing.

That they all seem to be zebra.