When Twilight and Applejack explore a mysterious cave on the edge of Sweet Apple Acres and things go awry, they are quite literally swept away into a world of darkness and ancient secrets, in a cave far beyond the reach of the Sun--where a relic of the past lives on and flourishes under the light of the Moon.

Deep within the heart of the earth, a colony of nocturnal ponies lives on who left Equestria in disgust after the defeat of the Night Mare at the hooves of Celestia. However, what will they think if they learn that their Princess has returned after a thousand years of waiting?

Inspired by DuoCartoonist's "Children of the Night".