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.
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.