Johann was one of Ashildr and Tomas' two sons, the other being Rue.
During the 14th century, Ashildr, using the alias "Alys", married Tomas and conceived their daughter, Essie, by accident. Shortly after Essie was born, Ashildr decided to have two more children, Johann and Rue. Ashildr intended for her children to comfort one another on the day she would leave them to protect her secret.
Ashildr, having become wary of Tomas's growing comments related to her immortality, decided to leave him and take their children with her on a boat to England in 1348, where Johann and his siblings caught the bubonic plague. Ashildr tried to decide which one of them to save with the second Mire repair kit given to her by the Twelfth Doctor, but ultimately decided to let all three of them die. (PROSE: The Triple Knife)