A Demon in the Council

From publisher blurb:

A demon took possession of Sir Stepan, one of the candidates for the new position in the council. The demon uses a magical artifact to influence all voters. It plans to spread its corruption once it obtains a powerful position in the city. However, Sir Harvey, the other candidate contending for the position suspects something is amiss. He does not dare tail Sir Stepan alone, but he is almost certain that following him is the way to reveal what caused his sudden change. So, he hires external aid to prevent his suspicions from reaching the wrong ears. Elections are in less than a moon, so there is no time to waste.