The Secret of the Dead Monks

In an abbey, 4 monks are found dead - all with black spots on their fingertips and on the tongue. Instead of extensive investigations, only more gruesome things happen in the abbey. Two other monks are found tortured in the vaulted cellar, and a woman was burned as a witch in the courtyard last night. The fact that no one takes care of the incidents seems highly suspicious to you and you start investigating secretly. The clues lead to the library in the highest tower of the abbey. You wait until the old eccentric librarian is gone and enter the mysterious rooms… Is the murderer hiding here?