Many dabble with the Occult at some stage. Those travelled and wise to the world know that there is a darker, unknown side to life. Necromancers toy on the fringes of the Occult energies to fuel their art, and those of zeal and religious fervor occasionally succumb to the darker side of their faith. Demons. Devils. Infernal realms. The Weird and unnatural. Elder beings you dare not awaken. All are the domain of the occult. Come; stop wading in the shallows and plunge into the depths! Fall to the darker art and discover what the Occult Brotherhood has to offer.
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?