Wytchwolde-Under-Ash, once a great Thorpe, was razed to the ground by the ruthless, and truth told more than slightly deranged, Porcelain Princess and her henchmen, the Purifiers. When the flames had at last subsided, and a kaleidoscope of swirling, dull-gray ash choked the sky, nine hundred acres of old growth iron spruce, black larch and weeping birch, was burned to utter cinders, along with the entire coven of witches comprising the Sisterhood of the Silver Teat.
Now, centuries later, the forests are somewhat re-grown, and the town of Foolswater stands where Wytchwolde-Under-Ash once did. It is said that even to this day, one can still find ashes in the otherwise potable well-water of this village. Once a year during the Winter Solstice, the “Ash-Wind” comes to Foolswater, a suffocating black cloud that passes quickly but leaves dead birds and animals in its wake, darkening the trees, and staining the sky with black snow. The inhabitants of the village know better than to be caught outside during the day-long Ash-Wind. Everyone is locked snugly inside, singing old hymns that curse and re-curse the burned witches who once called this place home.
A village where everyone goes blind sometime during puberty. What could be causing this? Perhaps something in their diet. Or some malignant gene. How would an entire village where only young childen can see function?
A vicious murder happens in a town that the party happens to pop in at. Under heavy suspicion as they are strangers, the party is forced to discover the perpetrator or have their reputations blackened, especially as more and more murders occur in the town, and mysteriously stop and restart when the party leaves to go kill off that evil necromancer who kidnapped the princess.
The only problem is that a demon, possessing one of the party, is the perpetrator. And the demon makes no signs that its living in the PC's head.
In fact, for all the party knows (except for the possessed person's), their companion is an evil murderer.
Do they try and execute their friend as he's a vicious murderer, and no evidence points to demonic posession? Or do they flee town with him, trusting him, and have their reputations destroyed?