|Poussin - The Plague at Ashod (WikiPaintings)|
The seat of the empire, the City, has served as the seat of power and the source of culture for ages untold. The hegemonic family's rule has been unchallenged. But now, plague stalks the city, and the Heavenly Family was one of the first to succumb. In the power vacuum that followed, lords stood dumbfounded, but only for a moment before they were at each other's throats.
Quickly, the disease raged unchecked, and the city descended into chaos, law fleeing to the hills with the nobles outside the now cursed city walls. The one accord the warring nobles made was to seal the city from the outside, hoping to save the surrounding lands by a great quarantine.
The PCs have been sealed within. Food dwindles, and many of those with arcane knowledge have turned to necromancy, either to preserve themselves from slow and painful death through un-death, or to make use of city's now greatest resource: corpses uncountable. This is D&D as survival horror, with "treasure" often being another meal. Like insanity in Call of Cthulhu, death, while not fated, would be to some degree expected at some point.
In addition to some human classes, several types of B/X-style race as class undead would be playable, and would probably have some sort of humanity stat (perhaps shoehorned into wisdom) which would be the undead analogue of constitution/hunger for the still living. Negotiating the factions, building new power structures within the anarchic confines of the City, and maintaining enough resources to survive would be the primary objectives of gameplay. Perhaps playing an undead would need to be unlocked first, by finding and performing the appropriate necromancy.