The soul-cleansing pool

A device that drains the evil out of a person and stores it unsafely.

The abbey at Mospid town owns a curious device, a shallow pool connected by underground pipes to a man-sized glass globe. Anyone who enters the pool while it's active has all their evil thoughts and impulses sucked down the drain and into the globe forever. Once in the water, the suction applied to their contained evil keeps them from stepping out until the transformation is finished and the evil is gone. 

People who have been through the process are placid, cooperative, and completely unable to make decisions for themselves. Perfect for cloistered monks. They work the abbey's fields and dairy herds, producing the famous Mospid cheese the abbey earns so much of its income from. The abbey also accepts a small fee from the local magistrate for every malcontent they turn into a productive member of society. As the abbot himself would say, the order is doubly blessed. (And any rumours about converts working themselves to death while their careless minders sleep or gamble are vile lies and perilously close to blasphemy.)

If there's one fly in his ointment, it's the glass globe. During the first year the pool was used, the evil it contained was just a thin haze. Now it's a roiling dark cloud crackling with electrical discharges. Sometimes the globe rocks on its stone base. The abbot has it strapped in place with ropes and nets while the pool's not in use. While it is, and there are visitors watching, he has men standing around it ready to steady it as needed. They don't like touching it. It feels like doing something shameful.

Clearly the amount of evil being stored now is a problem. The abbot has been experimenting with draining evil into pilgrims. These pilgrims come to the abbey for hospice care while they die, conveniently wiping out the burden of second-hand evil. The process is difficult. Like trying to take a cupful of water from a high pressure hose. Excess evil has been leaking out into the surroundings. It causes nightmares, sickens livestock and sours milk. The abbot suspended the experiments when hostile half-human creatures started to appear in nearby forests.

The obvious solution is to close the pool down and pay someone to lose the globe out past the edges of civilisation. The abbot vetoed that idea. Mospid Abbey depends on on the money and workforce the pool generates. Instead, he's been paying wizards and sages to secretly travel to Mospid and examine the globe. None of them have had any useful suggestions yet, so he has them processed through the pool to keep the secret.

The problem is coming to a crisis point. It's the perfect time for a party of adventurers to make things better worse different.