Something about the spell went horribly wrong. Celestia doesn't even remember what the spell was originally meant to do, but whatever it was, it failed. Instead, it changed her. It warped space and time around her and when she woke up, she was a very young filly again.

The tiny alicorn sits in her over-sized throne, wide-eyed and excited to be the princess of an entire kingdom.

"Hewwo!" she shouts, waving her tiny hoof. "I'm Pwincess Cewestia! May the sun bwess you!"