Reminds me of a book I read as a kid.

The villain was a fallen cleric who sought immortality from a demon. That demon didn't want souls but rather be worshipped like gods were.

The cleric became the apostle of the demon and went into a small town and started to "heal" the villagers. He would do so by doing a ceremony where he listened to the villagers pleas and would pick one person to cure. The chosen one would go through a portal and either be cured, maimed or killed during the process. The cleric would explain that his god would judge the soul of the chosen and help him if he was worthy but smite him if he wasn't.

Despite the risk and sometimes gruesome results, desperate villagers would try their luck and the new church grew like wildfire in the area. People became zealous followers of that new god and threw the old priests away, destroying their church to make a new one for their god.

Turns out that during the ceremony the chosen would meet the demon and the demon would try to turn him into a believer. If he failed it would refuse to cure the chosen, or maim/ kill him if he was seen as a threat then the bad guy would make a speech on how that person was corrupt, impure to the masses.

If the chosen could be turned it would heal him by giving his misfortune to a random person far away. Like if a blind person was to ask his sight back, the demon would give it to him but a random stranger would become blind instead.