Such a bloodied claymore has history way back through lines of father and son. In fact only one place near the handle still holds a metal tone, because of a magical barrier, and it has ingraved in it words that forever speak out to the swords bearer. Those words, like a guilty memory, can never be forgot.
“Thou who shall kill a sons father, shall then be killed by a fathers son.”
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?