Later I heard rumours, dark rumours
~ Wolfstan about Osmund

Osmund is the main protagonist turned secondary antagonist of the horror movie Black Death starring Sean Bean and he is played by Eddie Redmayne. He is technically a good man turned evil. He is a humble monk in the opening events of the film during the triumph of the Black Death and Osmund is in love with Averill, a young woman who had sought shelter in his abbey. Osmund is trying to protect Averill from the plague, and he is one day visited by noble warrior Ulric, (played by Sean Bean) who says he knows of a sanctuary from the plague, and that the plague will not reach the village he knows of. Osmund volunteers to leave the abbey and accompany Ulric and his men on the quest. One of the men, Wolfstan, becomes Osmund's mentor.

Before leaving, Osmund had told Averill to hide in the woods, and he would meet her when he called her. One morning Osmund goes to call her, but then he finds her brutally murdered, and lying dead on the forest floor. This is the start of Osmund's dementia and illness, because he believes that his whole life was now a waste of time.

Osmund accompanies Ulric and his men to the village and they are welcomed after a fashion, and the village leaders are Langiva, an alleged witch, and Hob who is her henchman. They threaten to make them abandon God, otherwise they will die. Osmund is told by Langiva that his lover, Averill, is alive, and is in the hut yonder. He goes nervously in, and sees Averill stumbling about in confusion as if demented. Osmund grips her and says that she is a monstrosity as God wouldn't allow anything to return from the dead, and Osmund says he'll see his lover again in Purgatory and stabs her. With Averill dead, seemingly twice, Osmund goes beserk as Ulric is chained and torn to pieces by horses. The rest of Ulric's men kill the villagers, who escape into the wild.

Osmund follows Langvine the witch into the swamp and he says that God is the only way, but Langvine says that Averill was never dead, she drugged her before she got to her village and now she was just comatose, but awoke, hence her confusion. Langiva says to Osmund that maybe he should look to God for answers if God really knows it all. Then, Langiva vanishes into the fog leaving Osmund traumatized and suicidally insane.

Osmund is taken back to the abbey by Wolfstan, who expresses concern for his safety and his mentality. Osmund does recover, but becomes a fully evil, sinister, misognyistic and hateful fanatic, he makes it his duty to rid the world of witches, and tortures women before killing them, seeing Langiva in the faces of the women he arrests for execution.