Art by Aleksi Briclot. Check out his myspace page.
Setting: A world ravaged by a taint of evil has not seen the sun for 1,000 years. Small outposts of humanity nestle together to forge a wall against the hordes of demons and undead that plague the land. Lead by vile Lord of Darkness, the armies of the apocolypse throw themselves against mankind's defenses daily, in an attempt to slowly erode the population.
Expendable and replenishable undead make the greatest bulk of the armies.
Rules: Just about any SENTIENT race is going to be playable AND living within the confines of the world's largest cities. Classes like bards and rogues are less likely to flourish in a world where everyone must work together to survive. Evil characters are almost unheard of. There is probably an obundence of weapons and armor, although none of it in great shape. Finding arms and armor is easy; finding quality merchandise is high impossible. The Free Kingdoms from the Accordlands (Warlord) have the look of a disheveled army for a product like this.
World Logic: People have learned to use fire to destroy wights, zombies and all manner of undead, so nothing could come back. Even their own dead would be cremated. Travel would be cut off and food sources are scarce. Magic would be used as a fundemental way of feeding the armies of humans and so on. Elves, most likely, would have more control, as they require less food to survive and can use their surplus as "wealth." Wizards are going to be hard to play considering how hard it is to share knowledge with the outside world.
Missions: Adventures would range from defending the city to armed escorts to raids to retreiving the dead to doom patrols to hit and run squads to recon to assassinations against specific Lords of Darkness
DM Advice: Keep the adventures focused on the evil around the PCs. Minutia like shopping and tavern hopping doesn't belong in a horror campaign. Incorporate real horror rules and keep the PCs on their toes. Sanity checks ala Cthulhu would be fitting even, especially if the adventure has other worldly gods.
Feel free to expand on this.