A thousand years ago, a long-forgotten disaster wracked Equestria, rendering its surface inhospitable to all but the most foolhardy of creatures. In the millennia since, the once unified and prosperous races and societies have devolved into fractured and competing tribes and factions, all trying to carve out their own piece of the Underworld for themselves.

Aspirant Twilight Sparkle is desperate for a chance to make a name for herself -- to become a full fledged Pathfinder, which would finally allow her to be free from the limits of Charlie Compound permanently. And if she manages to discover something that no other pony in the Erudite Enclave has, she'll be set for life.

However, in her quest for discovery, she may come across more than she bargained for. Unlikely allies, unforeseen enemies, and heretofore untold truths of the Underworld itself.

Unbeknownst to her, though -- that may be exactly what she's looking for. After all, the depths of discovery knows no bounds.