Johann was one of Ashildr and Tomas' two sons, the other being Rue.
After having Essie, Ashildr, who was using the alias "Alys", decided to have two more children, Johann and Rue, so they can comfort one another on the day Ashildr would have to leave the in order 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)