John of Gaunt, 1st Duke of Lancaster (1340 – 1399)
John of Gaunt was the fourth son of Edward III of England and one of the most influential nobles in England. He became the 1st Duke of Lancaster through marriage with Blanche, heiress of Lancaster in 1362. After Blanche’s death in 1371 he married Constance, daughter of Peter the Cruel and claimed the Castilian throne but failed to implement his claim. However, he married his daughter by Constance of Castile to the future Henry III of Castile in 1388 .
John of Gaunt was made Duke of Aquitaine in 1389 and helped to end the conflict between his brother Richard II of England and the hostile barons led by Thomas of Woodstock. He married his mistress Catherine Swynford in 1396, while their children (the Beauforts) were legitimized by King Richard II and the Church but barred from inheriting the English throne. However, Henry Tudor who descended from the line of the Beauforts won the English throne in 1485.