Nightmare Moon received an odd omen from the one who destroyed Equestria's past and now must contend with the possibility of her sister's anger coalescing into a true being while Nightmare Moon tries to retake Equestria from the King and Queen.
After her return, Princess Luna realizes that her sister has become the biggest princess -or any form of nobility for that matter- in Equestria's history. She's going to use her artsy-fartsy to make sure it stays that way.
Twilight Sparkle is curious of how the world was before ponies and other races came to be and asks Celestia on their and magic's origins. The answer is more absurd than she expected.
The spell trapping Nightmare Moon within the orbital satellite is tampered with, pushing her release past a thousand years into a time where alicorns are myth, technology has somewhat stagnated, and nobody knows who she is.