When Twilight finds herself urgently summoned to Canterlot by Princess Celestia, she knows something's terribly wrong. She's faced enemies far beyond her years but nothing could have prepared her for this. The skies have turned against the royal sisters. The sun and the moon have rebelled.

While racing to find the reason behind this, Twilight discovers more than she could have ever imagined...the story of Celestia's past, the ancient battles that have never truly ended...a darkness beyond all knowing that lies just beyond Equestria's borders.

As centuries-old secrets come to light, Twilight realizes that her new status as princess has bestowed upon her much more than just wings and a crown. Yet with each step closer to an answer, more questions arise. Why has Celestia barricaded herself inside the castle? Where does Luna keep disappearing to in the middle of the night?

What is "The Astral Vow"?