The last time Twilight saw Starlight Glimmer, she threatened to kill Rainbow Dash when she was just a filly and ruin Twilight's life by going back in time. Only, when she failed to find Starlight in the past and returned home, Equestria wasn't a ruined wasteland, Ponyville was a lot bigger than she remembered, and all these new ponies in the castle were talking about some big pony they call Empress.