The Witcher Erland of Larvik

After nearly a month of experiments the mages finally succeeded when one of the younger boys survived the transformation with his mind and body intact, becoming the first witcher. Less then a week later Erland was also mutated, with his hatred for the sorcerers keeping him alive. However, even two centuries later he would claim he never experienced such pain as on that operating table.
source: https://witcher.fandom.com/wiki/Erland_of_Larvik