The PCs come to place where a sidetrack branches off from the main road.
A man dressed in peasant garb stops the PCs and requests their aid. He claims that his village, which lies down the sidetrack, is sorely plagued and in need of brave adventures such as themselves.
If the PCs inquire further he states that the village priest is a demon in human form and that he has been performing unspeakable rituals in the name of his dark gods.
He asks the PCs to go to the village church and deal with the demonic cleric.
1. Evil in the Church
The village priest has indeed been replaced by some demonic or undead doppelganger. The creature is extremely tough and virtually impossible to kill in a straight-on fight.
The real priest is being held prisoner by powerful enchantments, kept alive only because the demon needs his life force to maintain its presence on the mundane plane.
The key is to break the enchantments, thereby releasing the priest and destroying the demon.
2. Petty Vengeance
The village priest is just that, a village priest.
A man of very little imagination he tends to take the teachings of the Holy Books a bit too literally and, as a result, imposes quite a lot of fines and penances for a variety of petty offences that most priests turn a blind eye to. This has, of course, made him deeply unpopular with the villagers.
The peasant, having had one penance too many imposed upon him, has finally had enough and decided to give the priest a taste of his own medicine. He figures that a bunch a violent monster-killers (enter the PCs) are just what he needs to give the priest a really good scare.
As above, the priest is simply a priest.
However the peasant is a demon seeking to spread as much mayhem as possible. Sending a group of honourable adventures to kill an upstanding village priest is just the sort of thing he finds entertaining.