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 stands weakened. Plagued by the political fallout of a princess' banishment. Ripe for exploitation by mortal and immortal alike, given enough leverage. Now, with precious few allies at her side, Princess Celestia must stave off those who would seek to rekindle the flames of war... and to bathe Equestria in darkness forever.
(Canon to seasons 1–3 only, Twilicorn-inclusive. Accounting for season 4 would require a complete overhaul!)