John of Gaunt was one of the sons of King Edward III and Philippa of Hainault. He was married three times, first to Blanche of Lancaster through whom he inherited the vast Lancaster estates. Second to Constance of Castile through whom he unsuccessfully tried to claim the Castilian throne. And finally Katherine Swynford, who had been his mistress for many years before their marriage.
After he died his lands were effectively stolen by King Richard II, leading John's son to raise a rebellion and eventually take the throne as King Henry IV.