A dark and lonely coast. The natives are a mistrustful sort, shunning strangers, shutting themselves up in their houses at night, whispering of foul things that lurk in the shadows. But there is one spot of light here, a small chapel with mysterious healing powers.
The chapel is one room, about 20 feet square, a grey, weathered wood, with a dirt floor. The floor is a couple feet below the bottom of the door, for it is the dirt itself that has the healing powers-- eating a small amount of the dirt will act as a gradual Greater Restoration, resoring one level/stat point per day, or a Regenerate that slowly takes effect over a week or so. The hermit who lives in the shrine implores the party to only take what they need, as the dirt loses potency within a few days of leaving the chapel, and warns the party against being out at night, inviting them to stay in the chapel.
As night falls, ghouls will appear and surround the chapel. The Hermit doesn't want to talk about them, but if pressed, will say there have always been a few around, but recently there have been a lot more. If the party chooses to investigate the next day, tracking the ghouls or a half-day searching the area will reveal an entrance to a partially flooded cave complex, inhabited by ghouls and a band of Kuo-toa. The ghouls will attack both the party and the Kuo-toa, although the Kuo-toa clerics will have used their turning to command as many as they can manage. The Kuo-toa will not immediately attack, but warn the party away. If questioned, they won't say much, but will let slip that they're only there temporarily.
In fact, the Kuo-toa are there to excavate and raise the gigantic evil skeleton of an ancient nameless evil. It is the presence of this skeleton which causes anyone in the caves or the land around to be raised as a ghoul d4 days after their death, just as if he were killed by a ghoul. The Kuo-toa have captured an earth elemental to speed the excavation, and in a large central room the skeleton is mostly uncovered, save for the skull and one leg. It is enormous: the rib cage alone is a good 20 by 30 feet, the bones are of strange proportions to humans, and detect as evil. The skeleton is bound in iron chains, with an arn padlock hanging down between the ribs, all of a scale with the skeleton, and unrusted, despite the age and the damp. The lock detects as good, and has a Magic Circle Against Evil effect around it, which extends up to any areas of the caves above it, and to the chapel, directly above. Kuo-toa clecics in this room can draw upon the power of the skeleton to add an extra 2d6 unholy damage to their lightning bolts against good characters.
In one of the submerged sections of the caves, in the head Kuo-toa priest's quarters, the party can find the key to the lock, as well as the vile tomes containing the rites the priest intends to use to control and bargain with the ancient evil. The tomes are bound in Kuo-toa skin, and will hit any good characters who read them with a negative level. The key is exceptionally heavy, weighing 50 pounds (-10 swim!), and will do one of those funny space-bending things if used on the lock, appearing to fit perfectly in both the lock and the hand using it.
If the party is sensible, they'll just off the Kuo-toa or steal the books and key to prevent the evil thing from coming back. They might also want to try to cover up the skeleton again-- this will return the ghoul creation to the previous, lesser rate. If they use the key without the evil rites, the skeleton will awaken, claw its way out of the caves (causing the ceiling to collapse, as the earthquake spell), and walk out into the sea. This will at least stop more ghouls from being created, though the party may want to deal with the unspeakable evil they've unleased on the world at some point. If the party wants to try the evil rites? Well. Give 'em a wish, to be carried out in the way that causes the most collateral damage, and then the thing starts giving orders, starting with sacrificing the hermit to it.
Oh: For some parties, the hermit being worried about the ghouls may not be enough of an adventure hook. I suggest having the party run into the slaughtered, eaten remnants of a group of Kuo-toa the next morning, with clues on them about the evil power below.
Props to China Mieville, the Santuario de Chimayo, and, of course, Lovecraft. Oh, and the Goonies, but not for anything direct.