In Inferno, you play a Demon Prince in Hell. Using free-form turns, you describe what your character does. Harvest Sin from whatever damned souls you may control, use that Sin to manipulate the very shape of Hell, build demons, send those demons to attack your enemies, communicate with the living and make pacts for their souls... whatever you can imagine, you can do it. The degree of success depends both on how well you can describe your actions, and of course, on what the other players do.
The world of Hell began as a blank slate, and then the Demon Princes awoke. Soon, they had carved Hell up into various fiefdoms, shattered the land and begun mighty wars across the many floating islands that were left. What mark will you leave upon the world of the damned?