Fifteen years ago, the ravening hordes of Tartarus invaded Equestria, laying waste to the unprepared kingdom. To spare the rest of Equestria, Celestia sacrificed herself, and the city of Canterlot.

Canterlot sank beneath the world, landing on the shores of the Unterzee, a vast underground ocean which separates the world of the living from the pits of Tartarus. To face the challenges of this new world, without the light of Celestia's sun to guide them, Canterloters of all walks of life took to the zee to carve out a place for themselves.

One of them, a former Royal Navy Captain, finds herself in the middle of the great game between the powers of the Unterzee, and finds that little is ever actually under anypony's control.

A Sunless Sea crossover