@Background Pony #58B7
This one is where Spike finds The Book of Secrets, hidden in Celestia and Luna’s old castle.
How did the Crystal Empire come to be? Where did the Changelings come from? How was Discord actually formed? What was Tirek and Scorpan’s old home called? Who was Sombra before becoming the cruel tyrant he is? What caused the magic that formed the Pony of Shadows? Who was the original owner of the alicorn amulet? Is there a secret civilization composed of only alicorns somewhere on Equus?
The Book of Secrets answers all of these questions and more