"The Apocalypse Horses are mythical beasts that are said to bring chaos everywhere they go. The four Horses – Death, Famine, Pestilence, and War – have always existed in the spirit world, and it has been stated in legend that the Horses will one day rise and bring down the rains of fire and chaos with them, bearing nothing but destruction . . ."

When Twilight Sparkle stumbles across this passage in a book about mythical creatures and legends, she confronts Princess Celestia, who shows her a horrific sight underneath Castle Equestria. Celestia assures Twilight that everything will be fine, but when the solar eclipse occurs the next day - a ceremony held in honor of harmony between the two sisters of day and night, Celestia and Luna - things get dangerous fast when the Horses break free from their captivity . . .