All spell casters gain their abilities through a form of demon possession. So in order to gain magical abilites, PCs must track down and trap demons. Of course there are some pretty powerful demons, if the demon is stronger than the PC, the PC could find themselves subjugated to the Demon's will.
An adventuress, Semma, has returned to her home village, but something has gone badly wrong: it's the furtive glances, the conversations that stop when she approaches, the childhood friends who now have no time for her. She returns to the town or city and enlists your party to help her find out what's going on.
The party must find out what is wrong: something (perhaps a Cthulhoid monster, perhaps a gang of vampires, perhaps just a bunch of bandits) is extorting obedience from the villagers by threatening their loved ones with at least death. Once the source is discovered, Semma and the party and a few brave yeomen (and women) must deal with the threat by finding its base and defeating it, and then deal with the remnant corrupted villagers who willingly served the Evil Force. These may prove to be the most difficult foe...