This seems as good a thread as any to post this. It's an idea for a small adventure I've had kicking around for a while.
The PCs enter an old mansion. Now, all seems normal at first.
Then, they start hearing spooky noises. Sounds of footsteps behind them. A glimpse out of the corner of their eye of some shadowy figure.
Then they enter the basement.
They find a torture room; some instruments even have gore still on them. Screams come from behind a door.
Behind that door is a body, recently dead. There is a lever on the wall. Pulling it reveals yet another passage down.
When the players get down there, they find an old study, covered in books with demonic names and skulls with melted candles on them, the whole bit. There is a note on the desk that reads:
"Congratulations! You have braved my best! Come back upstairs and we'll celebrate over a drink!
Signed- (Name of person here)
Now, at this point, everyone will relax and go "oh, it was just a prank, how nice."
When they re-enter the room with the body, it is gone. A bloody trail leads into the torture room, where several zombie/ghoul/other creatures are eating a man. The man appears to have been well-dressed, and there is a broken bottle of wine on the ground.
For CoC, I'd alter it so it's just him, dead on the floor, and his hair and eyes are blank white.
Proceed with scary/undead adventure from here.