From the introduction:
Breachers of the Bone Belfry stands out from other adventures for Four Against Darkness in two ways. First, it is set in the Netherworld. Second, and most importantly, is the fact that the characters in your party are dead.
Specifically, you are invited to play your former characters who have died in a dungeon - from those who suffered a medusa’s stone gaze, who fell to wicked traps, gnawing vermin or other grisly ends. The Netherworld being an unforgiving place, their soul-selves, enslaved by demons, are destined to be ground into soul cubes. This adventure offers them a chance, however slim, to escape that grisly fate.