When I saw Smol Twilight and not Big Twilight, I facepalmed. So, taking everything in from A New Generation, Tell Your Tale, and Make Your Mark; I present to you, THE PAST!
Twilight has been presented a prophecy, one that'll determine the future of Equestria. A new threat rises on the horizon and sometimes it's best to step aside and let future generations deal with it.
(Yes, I am disregarding the comics. One, because I don't want to read that nor do I know how to read it. Two, because it just seems wrong. Dunno how else to explain it.)
Oct 7 note/update: I forgot about my notes at the beginning of the chapter. I have deleted them now. No idea why nobody commented on that...