The class itself is very interesting. It attacks as a fighter and can use any weapon or piece of armor. It has d6 hit dice. Only lawful humans can be witchfinders. The start with a bonus on saving throws versus magic from chaotic casters.
What really makes the class interesting to me is its gritty inquisitor abilities. All witchfinders have a higher than average chance of extracting useful information through a day's torture. This starts at 60% and increases 2% per class level.
The other starting ability they get is quite unique. It is spread anxiety. There are no hard and fast, concrete rules for this. Essentially it's a tool for the GM'. Commoners will treat the witchfinder with fear and paranoia. They will avoid the witchfinder and placate him to avoid suspicion.
As they level up they gain the ability to cast the following spells as a cleric: detect magic, detect evil, protection from evil, bless, protection from evil (10 foot radius), and holy word.
In addition, at 9th level they can establish a malefizhaus. This is a witch-house that attracts low level witch hunters that will vow to protect the surrounding area. Historically, a malfiz haus was essentially a church torture buildings.
I really, really like this class. While it doesn't have a place in light-hearted games, it could lead to some really interesting games with a darker tone.
The Masks of Satan are fitting tools for aspiring witchfinders. Looking for an encounter centered around witchfinders? You should check out One Small Cut.