Very cool game!
the following spells need to be very limited or banned
CLERIC SPELLS
0TH....Create Water
1ST....Detect Undead, Hide from Undead, Purify Food and Drink
2ND....Augury, Consecrate
3RD....Create Food and Water, Remove Disease
xth.....and all monster summons (not much sense of horrer if you summon creatures to draw out the undead, check the stairway, etc.
perhaps creating food / water fails because the whole region is tainted?
same vein, purify food & drink eliminates all food/drink issues, same as above?
detect undead spoils surprise undead
remove disease removes a great fear that undead injuries should insure
all divinations should be filtered with a force, much like ravenloft. sure knowledge kills fear
zombies will quickly become boring / unscary. what if undead feed on undead if they don't find another food source after a long enough wait? And the eater of the dead becomes something...darker. After having fed on the dead flesh of countless companions, edgar the zombie became a wight. So the undead reduce in numbers somewhat, but you have an excuse for the undead to rise in power to match the players. Also, even after some have risen, some would still be plain zombies.
disease - often, and painful. infected bites. tainted food and water.
pace the game 3 games pressure, 1 game hope, 3 games pressure, etc. after a while, continous pressure breeds indifference. also, try different types of pressure/horror (you found a small group and stay with them in the tiny shelter, but one makes a dark pact to become undead rather then be eventually eaten. he begins to sabatoge...)
have magic suffer/weaken as well - rings of fireball negate a lot of the fear. if magic is rare, and almost always limited charges (resources, resources, resources). no bags of holding or similar devices.
followers and leadership should be okay - the pc's are become responsible for the last bastion of mankind. do you want to take all of your warriors into the next game, your town may die. even if they get a 'home', limited resources will always pull them back out for more. also, with so few people alive, you will eventually need to have a source to replace a dead pc.
again, cool game, much fun! if magic is rare/hard to find, people will have to settle for alchemical (and herbal) equipment. it becomes treasure. you may find my herbalism posting in home brew useful (or not). have fun scaring the players!