In the 13th century, three women are accused of witchcraft and meet a grim fate at the hands of a priest. One of the women claims to be a witch under the church's persuasion, while another openly denies her innocence and curses the priest. He proceeds to hang them and commands the guards to hoist their bodies up again for a ritual aimed at ensuring they never arise from the dead; however, the guards dismiss his orders, declaring them dead enough. Terrified that they might resurrect, the priest sneaks back under the cover of night to perform the ritual himself. He completes the rite successfully for the first two bodies, but the third, who identifies herself as a herbalist, emerges with a demonic form and slays the priest.
Fast forward to the 14th century, where Teutonic Knights Behmen Von Bleibruck and Felson are deep in the trenches of a crusade, enduring numerous battles throughout the 1330s including the infamous Smyrniote crusades. After witnessing the massacre of civilians during the 1344 siege of Smyrna, they courageously decide to abandon the Order and return to Austria.
While traversing through Styria, they are confronted with the grim reality of the Black Death, which has ravaged the Holy Roman Empire. Upon reaching the medieval town of Marburg, they attempt to hide their identities as deserters. However, the crest on Behmen’s sword reveals their true status, leading to their arrest. They are brought before Cardinal D'Ambroise (played by Christopher Lee), who is also stricken by the plague.
The Cardinal tasks the knights with escorting a girl suspected of witchcraft—the alleged cause of the plague—to a distant monastery where elite monks can ascertain her true nature. If found guilty, the monks possess the knowledge of a sacred ritual capable of nullifying her powers and halting the rampant plague that threatens all of Europe. The two knights agree to the mission, provided that she receives a fair trial and that their desertion charges are rescinded, a condition the Cardinal accepts. They are joined by a priest named Debelzeq; Kay Von Wollenbarth (portrayed by Robert Sheehan), a young altar boy longing to be a knight like his late father; Eckhart (Ulrich Thomsen), whose family fell victim to the plague; and the seasoned trickster Hagamar (Stephen Graham), who guides them to the monastery in hopes of obtaining a pardon for his misdeeds. The witch, later revealed as Anna (played by Claire Foy), initially harbors animosity for Debelzeq but starts to form a bond with Behmen.
As the group sets up camp for the night, Eckhart takes the first watch over Anna. After some time, Debelzeq comes to take over. Eckhart shares tales of his daughter Mila, who bore a resemblance to Anna. However, when Anna perceives that she might be left alone with Debelzeq, her fears manifest into a violent outburst. In a desperate attempt to escape, she seizes Eckhart's key and absconds to a nearby village.
The subsequent manhunt leads them to a mass grave where Eckhart is haunted by visions of his deceased daughter. Chasing after these disturbing apparitions, he inadvertently impales himself on Kay’s sword and dies. When the group captures Anna once more, she tearfully explains her flight was solely due to her fear of Debelzeq. Nonetheless, suspicion begins to shroud Anna’s presence within the group. During a perilous crossing of a dilapidated rope bridge, Anna showcases an unnatural strength by saving Kay from falling to his death, heightening the stakes.
As they delve deeper into the ominous forest known as Wormwood, Hagamar’s treachery emerges as he attempts to eliminate Anna, but his plan is thwarted by the others. Soon after, Anna seemingly calls forth wolves that pursue the group, leading to Hagamar's demise. An enraged Behmen is halted by Debelzeq and Felson, who remind him that the monastery is now within their sights.
Upon their arrival, they discover the monastery's monks have all succumbed to the plague, yet they are fortunate to find the Key of Solomon, an ancient tome filled with divine exorcism rituals to vanquish evil. Debelzeq starts a ritual against Anna, but as Anna recounts Behmen's past Crusade actions with uncanny accuracy, Debelzeq realizes her accusations may be unfounded, leading him into a frantic exorcism instead.
Just then, the demon controlling Anna unveils itself, melting the cage’s metal bars. It easily overpowers the knights until Debelzeq douses it with holy water, compelling it to flee. In realization of the demon's intentions—to destroy the crucial book so that it may remain unfettered—the knights race against time. They locate a chamber where monks were reproducing the book, but the demon reappears, decimating the copies and seizing the corpses of the dead monks as its combatants. A fierce battle ensues as Debelzeq continues the exorcism, but tragically loses his life in the process, followed shortly by Felson.
Kay seizes the Book and strives to conclude the ritual while Behmen remains engaged in combat with the demon. Fatally wounded, Behmen implores Kay to safeguard Anna just before he succumbs to his injuries. With a heavy heart, Kay and Anna pay their respects to their fallen comrades, with Anna requesting tales of the brave men who came to her rescue. Finally, armed with the book, they depart from the monastery, their fates forever altered.
