An old witcher appears at the home of young Geralt's parents, wanting to take the boy away, as promised to him long ago under the so-called law of surprise. The father stands up for his son, but is quickly overpowered. Geralt ends up in the mountain fortress of Kaer Morhen. Here, young candidates have long been subjected to magical trials, mutations, and training designed to transform them into witchers—reliable killing machines. Only one in five boys survives all the trials. Upon seeing the boy, the priest Vesemir decides to subject him to experiments that have never been tried before.
