Katherine Swynford, widow of John of Gaunt
Katherine Swynford, Duchess of Lancaster (born Katherine de Roet, c. 1350 – 10 May 1403), also spelled Katharine or Catherine, was the third wife of John of Gaunt, Duke of Lancaster, a son of King Edward III. She had been the Duke's lover for many years before their marriage. The couple's children, born before the marriage, were later legitimised during the reign of the Duke's nephew, Richard II. When the Duke's son from his first marriage overthrew Richard, becoming Henry IV, he introduced a provision that neither they nor their descendants could ever claim the throne of England; however, the legitimacy for all rights was a parliamentary statute that Henry IV lacked the authority to amend.
Their descendants were members of the Beaufort family, which played a major role in the Wars of the Roses. Henry VII, who became King of England in 1485, derived his claim to the throne from his mother, Margaret Beaufort, who was a great-granddaughter of Gaunt and Swynford. His legal claim to the throne, therefore, was through a cognatic and previously illegitimate line. Henry's first action was to declare himself king "by right of conquest" retroactively from 21 August 1485, the day before his army defeated King Richard III at the Battle of Bosworth.