From the kickstarter:
Heart is a game set in the same world as the Spire RPG - in fact, it’s set underneath the city of Spire itself in the mad, shifting chasms and ruins known as the Heart.
From the back of the book:
Mages hooked on illegal magic, desperate for another hit.
Ruins ancient beyond reckoning, overgrown with bone and sinew.
A sunken moon, fecund and glorious, that calls the faithful ever-deeper.
Glyph-marked bees that swarm within you and seal your madness up with wax.
Downtrodden mercenaries bearing the weight of a lost regiment on their shoulders.
Regal witches who channel unearthly powers through their own diseased blood.
Vagabond knights wearing armour bristling with barely-understood machinery.
Chthonic angels that sing a screeching, scraping song of rust and ashes.
A red wet heaven slumbering fitful under the city of spire.
This is the heart: the city beneath.
Heart: The City Beneath is a tabletop roleplaying game about delving into a nightmare undercity that will give you everything you’ve ever dreamed of - or kill you in the process. It is a dungeon-crawling, story-forward tabletop RPG that focuses on what characters have to lose in pursuit of their dreams in the chaotic darkness beneath the world.