Merten is a witcher from the School of the Manticore and only mentioned in the Blood and Wine expansion. At one point, he ended up in prison while in Toussaint, where he befriended a priest of Lebioda. Having a change of heart and wanting to renounce his former life, once he was out of prison he set out on a pilgrimage, leaving a diagram of his school's gear at each spot before finally reaching the lake and tossing the rest in.