Great thread! I love horror games and creepy adventures, and some great ideas have been posted. Here are a few I have used over the years.
- D&D adventure: a group of humans who were followers of a prophet that warned of a coming catastrophe had built a large underground complex to weather the coming doom. Problem was that the badness they were trying to avoid (demonic incursion) found them, and the inhabitants were attacked and butchered by beastmen and demons. Before they left, the beastmen defiled the whole complex, and draped/hung/positioned bodies in gruesome and blasphemous ways. The PCs come into this complex 400 years later, and due to the manner of their deaths, many uneasy spirits roam the area. Not spirits in a game mechanics sense, just the souls of those slain that couldn't move on. There were occasional sounds of voices, footfalls, drafts, feeling like someone watching you, and light tugs on clothing that no discernable thing had caused. When the characters got to the main living hub in the subterranean chambers, they split up to search the area, and one lady who gamed with us and played a bard went into what was formerly the school for children. While in there, she heard a small child's voice which seemed to originate near her but sounded distant say "I'm so cold". After trying to locate the voice for a couple minutes with it repeating the same sentence, she said "what do you want sweetie?" The voice said "I'm so cold, I just want to be warm". The bard knelt down in the floor and opened her arms like she was going to hug someone, and suddenly she felt a press of small cold bodies all around her, enveloping her. Next thing she knew, she was waking up and the party cleric was casting Restorations on her (she had been Str drained), but the ghostly children followed her around the rest of the time they were in the complex, and warned the bard of danger several times, saving the group from being buried in a cave-in, ambushed by a demonic presence, and even helped them locate the object they went there to find. This really freaked the rest of the group out, especially since the ghosts didn't manifest visibly. After they left the area, the ghost of one little girl stayed with the bard, and occasionally would give her advice during an adventure (always in a somewhat disturbing way), and called her "auntie Parthenia".
- D&D adventure: over the course of several adventures, the PCs heard strange noises coming from the ground, walls, floors, etc in areas they were in. Inspection of the area revealed nothing but weathered and worm-eaten material, but there was a faint magical aura when they finally decided to detect magic. Later, the group ran into a madman, who claimed that the "worms of the deep" were always listening and watching. The PCs didn't start to really put things together until later, when they ran across an old enemy that was a priest of the god of decay and death that seemed to know every detail of their plans and secrets. They later figured out that he was using some kind of small, immature wormlike creatures to spy on them, and he himself was in thrall to some kind of subterranean wormlike beasts that sought to subvert and dominate the surface dwellers (very Lovecraftian type of creature). The priest was also infecting people he kidnapped with the immature larva, and those infected were controlled by telepathic links with the maters before they were devoured from the inside out, giving rise to strange beasts that caused the environment and reality to warp and change, causing madness. Once they realized what was going on, be PCs became paranoid about finding areas the worms couldn't get to to overhear them, and were very fearful of the "masters". They finally managed to kill the priest and his spawning pit, but who or what the "masters" are they have never figured out, and remain fearful that other cells of the "Cult of the Worm" as they termed it, might be out there.
- D&D adventure: This started off as a random encounter, which I elaborated on the fly. While traveling along an old road near sundown, the party sees a light like a lantern up on a ridge. The light stays in one place for a few minutes, bobbing and weaving, and occasionally flashing on and off, before moving along the ridge. The party decides to follow the light and does so for about two miles, back into the foothills of some mountains, eventually arriving at a ruined abbey. Even before they began exploring, the paladin said "I have a really bad feeling about this place, its unclean." They explored the abbey for about 20 minutes, until the ranger found the large bell laying in the bottom of the old tower and smacked it with his warhammer. A few minutes later, crows begin to flock to the abbey by the hundreds, cawing loudly and watching the PCs. Over all the racket the birds are raising, the rogue in the group think he hears chanting, very faintly and muffled. The group decides to leave, not even having explored the 1/3 of the abbey left above ground. They make it to a roadside inn an hour or so after sundown, and after a few drinks the ranger asks the innkeeper if he knows about a ruined abbey nearby. The innkeep goes pale, and said he did, that an order of monks used to live there about 100 years ago until it was discovered they were demon-worshippers and cannibals, and would lure travelers to their abbey by having one of the monks wait on a ridge near the road and lead them back. Once the monks finished their feast, they would ring the bell and place the discarded bits of their victims in the courtyard, where flocks of hungry fiendish crows would descend to devour the remains. The order was censured and destroyed by their parent church as blasphemous, but strage tales still speak of lights on the ridge, and that the ghosts of the former monks still exist there, attempting to lure travelers back to their doom for their infernal masters. All the players at the table looked at each other with looks of fear and disgust on their faces, and the paladin's player said "I KNEW that place was just wrong." Oddly enough, he NEVER tried to detect evil there.
I have lots more, and I'll post some more later if there is continued interest in this thread.