This story is a sequel to Celestial Bodies

In the cold dark a bright tree burns, and turning in orbit around this golden star are the planetary ponies that sleep in the starry night. Upon the flank of Sol Celestia is the Citadel of the Mark, the oldest and largest city on both her, and her valence moon-sister. Higher up is the dock-side city of Aesir-Bilröst, founded by the unicorn seer Starswirl after receiving a vision from the sun-mother.

Explorers beneath the Rainbow-Tail have discovered a well and the scholars from the dock-side city can not resist the urge to map it out, but first it must be pried open. Starswirl and his apprentice, Celeste, make the journey down to solve this issue and chart the unknown, and in so doing advance a prophecy glimpsed by Starswirl during a long-ago war.