West of Dead

As far as I can tell you need to beat the game once before these start showing up as I never got any to proc during my first partial playthrough or second full.

You are placed ina room with an introduction to them so you cant miss them same as the Preacher fights.
My only tips would be to search a level completely before moving on as sometimes they can spawn in a room or when leaving the Witches Chambers you can be placed straight into a fight with them.

I got it in the first level after reaching the farm (5th level) on my previous run so I think you need to beat the preacher (4th level, end of 1st chapter) for these to show up