One morning, Twilight received a letter from The Crystal Empire.
It said: "Dear Twilight,
I need a favor from you and your friends. I am curious as to why Flurry was an alicorn when I gave birth to her. Can you please find an answer for me? That would be most sweet of you. Signed, Cadence"
Twilight knew what Cadence meant, and she gathered her friends.
"I think there might be an old book at Celestia and Luna's old castle with an answer to that.", said Rarity.
"I think you are right.", said Twilight in return.
So, they all began the journey to the ancient ruins of The Castle of The Two Sisters, not knowing what they would uncover.