Jad Karadin was a retired witcher of the Cat School. He lived in Novigrad with his wife and two adopted children. Karadin formerly led a band of assassins, of which Selyse, Lund, Hammond and Vienne were members. Whether there were more former members alive at the time Karadin is encountered remains unknown.
The witcher Lambert, whom Geralt of Rivia encounters shortly after taking on a contract for an ekimmara issued by one of Lund's guardsmen, had a personal interest in finding Karadin. Having credible information that the now-disbanded gang had killed Aiden, a witcher of the Cat School, and a good friend of Lambert. With Geralt of Rivia's help they then try to find Karadin to exact revenge for the murder of the fellow witcher.
The duo manages to track Karadin, found living as a wealthy merchant in Novigrad. Karadin claims he had given up the witcher trade and his criminal activities to start a new life with his wife and two adopted children. He tells them that Aiden's death was an accident and that he greatly regrets it. Lambert refuses to believe him, and claims to have knowledge of Karadin working in the slave trade.
In the end, Geralt of Rivia must decide whether he helps Lambert follow through with his revenge or if he does not want to back Lambert in his decision. If Geralt decides to help Lambert, a fight ensues where Karadin will meet his end. If Geralt chooses to take no part in Lambert's revenge, he tells Lambert that "he may do with Karadin as he wishes", but he will take no part in it.