Season of the Witch

In Villach in the 13th century, three women are accused of witchcraft by a priest. While one claims to be a witch through persuasion from the church, one denies it, another does neither and curses the priest. He orders them hanged and drowned. That done, he urges the guards to pull them back up for a ritual to make sure the so-called witches never come back to life. The guards refuse, claiming they are dead enough. The priest returns late at night to perform the ritual. One of the corpses, one of a hag with a blind eye suffers a seizure but dies, While another corpse, a younger woman takes on a demonic appearance and kills the priest.
In the 14th century, Teutonic Knights Behmen von Bleibruck (Cage) and Felson (Perlman) are engaged on a crusade, taking part in several different battles throughout the 1330s and eventually in the Smyrniote crusades. After witnessing the massacre of civilians during the 1344 capture of Smyrna, the two knights desert the Order and the crusade and return to Austria.
While traveling through Styria, Behmen and Felson encounter the grotesque sight of people infected with the Black Death and discover that the Holy Roman Empire has been swept by the plague. They enter an unnamed town, trying to conceal their identity as deserters, but are revealed as knights by the crest on Behmen's sword. They are arrested and taken to Cardinal D'Ambroise, who is infected with the plague. The Cardinal asks the knights to escort an alleged witch suspected of causing the plague to a remote monastery, where an elite group of monks reside. These monks can determine if the girl is truly a witch, and if she is found guilty, know a sacred ritual to cancel her powers and stop the plague that is devastating Europe. The two knights agree under the condition she be given a fair trial and that the charges of desertion against them be dropped. The Cardinal agrees and they set out accompanied by a priest, Debelzaq; a young altar boy, Kay von Wollenbarth, who wants to become a knight like his deceased father; a knight, Johann Eckhardt, whose family was killed by the plague; and the well-traveled swindler Hagamar, who will serve as their guide to the monastery in return for a pardon. The witch, a young girl later identified as Anna from Marburg, shows hatred towards Debelzaq and forms a bond with Behmen.
Shortly after setting off, the group camp for the night. Anna becomes hysterical at the prospect of being left alone with Debelzaq. She attacks him and grabs his key to the cage, then escapes, fleeing toward a nearby village. The search for her leads the group to a mass grave, where Eckhart has visions of his dead daughter. Chasing the visions, he impales himself on Kay's sword and dies. When they recapture her, the tearful Anna explains that she only ran away for fear of Debelzaq. However the group gradually becomes less trusting of her. They cross a rickety rope bridge, where Anna saves Kay from falling to his death by grabbing him with one hand, showing unnatural strength. The group enters the dark forest called Wormwood, where Hagamar attempts to kill Anna so that the group can go home, only to be stopped by the others. Anna appears to summon monstrous wolves, which chase the group and kill Hagamar. An enraged Behmen tries to kill Anna, but is stopped by Debelzaq and Felson, who point out that the monastery is in sight.
At the monastery, the men find that all the monks have been killed by the plague, but locate the Key of Solomon, an ancient book filled with holy rituals used to defeat evil. The men confront Anna, and Debelzaq begins to perform a ritual used on witches. However, as Anna begins precisely recounting Behmen's past actions during the Crusades, Debelzaq realizes that she is not a witch, and begins frantically performing an exorcism. However, the demon that is possessing Anna reveals himself and melts the metal of the cage. He fights off the knights effortlessly, but when Debelzaq throws a vial of holy water on him, the demon flies out of sight. As the men search for the demon, they realize he is not trying to escape, but to destroy the book so that nothing can stop his powers. They find a room where the monks were writing copies of the book, where the demon, revealing himself to be Baal, destroys the copies and possesses the dead monks' bodies to use as weapons. The three men fight the possessed monks while Debelzaq continues the exorcism ritual. During the fight, Baal/Anna breaks Debelzaq's neck, then kills Felson. Kay picks up the book and continues the ritual, while Behmen continues fighting Baal/Anna. Behmen is mortally wounded during the fight, but Kay is able to finish the ritual and Baal is expelled from Anna's body, freeing her. Behmen asks Kay to keep Anna safe and then dies of his wounds.
Kay and Anna bury their fallen friends. Anna asks Kay to tell her about the men who saved her. They depart from the monastery with the book.

Question: What killed all the monastery monks?
Answer: Plague