Stressed by the growing rift between herself and her sister, Princess Celestia confides in her brightest student of the age. That fateful night, young Princess Luna's bitter hatred boils over, instigating a clash that not only goes down in legend, but shakes the very foundation of Equestria... and divides the diarchy for a thousand years.
Sadly, for those left behind, the nightmare doesn't end there. The kingdom's weakened state, plagued by the political fallout of a princess' banishment, proves ripe for exploitation... and there are those, mortal and immortal alike, who stand poised to thrust Equestria into the depths of war once again.
Princess Celestia, and her closest friends, have a dark future ahead of them.
(Canon to seasons 1–3 only. As of the season 4 premiere, the AU tag has been added. I have no problem with the developments presented to us by the new season, but the fact remains that this story is reliant on its own internal continuity. Otherwise, I would need to effect a complete overhaul from the ground up.)