During a friendly, casual visit to Canterlot Palace, Twilight asks Celestia a question many ponies have likely pondered for years.

Why is she a Princess and not a Queen?

And Twilight receives her answer in the form of a story, long lost to the passage of time. A story about the true Queen of Equestria, an alicorn known to be as the first ever created. Her name, Queen Athena, the Queen of Harmony.