The Secret of the Monks

Away from the cities, 4 dead monks were found in a remote abbey. They all had black fingertips and tongue tips. The locality was in uproar and out of fear of the devil, they tortured brothers of the abbey or burned single women accused of witchcraft. As you travel through the village, you hear about the incidents and want to take a closer look. It cannot be a coincidence that the 4 dead monks were all last seen in the priest's study room? You do not believe in the devil, much more you fear that the death of the monks was murder. The prayer time is approaching but you want to use the absence of the monks to find out what happened here. You gain access to the mysterious rooms in the most northern tower of the abbey. Can you expose the murderer?