Haven Town is facing total annihilation at the hands (and teeth) of a horde of monsters from beyond the Shadowrift.
You the heroes must band together to drive them back. To do this, you will need powerful spells, skills, attacks and loot. When the game begins, you are a basic hero; you can explore and fight. Lucky for you, this is a deckbuilding game! You can buy new cards to add to your deck - cards which will define you as an adventurer and complement the strengths of your fellow heroes.
Unlike other deckbuilders, there is constant interaction with your fellow players, as you figure out who will gain which benefit from the limited supply of townsfolk, offer your coin to help construct walls, and seek healing from anyone who's learned such magic.
Shadowrift also features monsters that do not merely sit in play waiting to be slain - If you leave them alone they will rip Haven Town asunder. They'll kill people, break walls, and kick your heroes in the face. Combat with them is intuitive (though frequently painful).
For defeating a monster, heroes gain Heroism, a simple, consistent boost to their power that makes them better at anything they undertake. Since the monsters won't stop coming until the last Shadowrift is sealed or the town has been built into a mighty fortress, you'll need every boost you can take.