Marta was a bruxa and the former wife of Jakob Ornstine.
Biography
Marta and Jakob were travelling through a dangerous land, when they encountered a bruxa. While attempting to flee, the bruxa grabbed Marta and transformed her into one of her own. Jakob and Marta then stayed connected as she would follow him around Caed Dhu, and would attack all but him, so he started to tell lost travelers not to go in the forest.
When Jakob met Geralt of Rivia, the witcher grew anxious seeing the bruxa on a nearby hill.