1000 years can change any story. What if the version that Twilight read was wrong? What if Luna and Celestia had a war and Celestia was simply the victor? What would happen to the land of Equestria then?
This is slightly inspired by the comic My Little Pony: Friendship is Betrayal. (don't worry, I won't get too dark. I just read it and thought, hmm, that would be interesting to write about.)