English noblewoman
Isabel de Beaumont, Duchess of Lancaster, of the House of Brienne (c. 1318 – 1361) was an English noblewoman. She was the youngest daughter and child of Henry de Beaumont, Earl of Buchan and Alice Comyn, Countess of Buchan.
Family.
Isabel was born in about 1318. She had nine siblings, including John de Beaumont, 2nd Lord Beaumont. Isabel's paternal grandparents were Louis of Brienne, Viscount de Beaumont, and Agnes, Viscountess de Beaumont. Her maternal grandparents were Alexander Comyn, Sheriff of Aberdeen and Joan le Latimer. Louis of Brienne was a younger son of John of Brienne by his third wife, Berengaria of Leon while Alexander Comyn was a younger brother of John Comyn, Earl of Buchan.
Marriage and children.
She married Henry of Grosmont, 1st Duke of Lancaster in 1330, but lived apart from her husband for a few years before consummation of the marriage due to her age. As her husband's mother had died before the marriage, she became the senior lady of the Lancaster household. 
Isabel accompanied her husband on some of his diplomatic travels to the continent, such as to the Low Countries in 1341 and Gascony in 1345. In 1351, he was awarded the title Duke of Lancaster and Isabel became Duchess of Lancaster, the first woman in England to hold the title. 
Isabel and Henry had daughters who would eventually inherit their father's estates: 
Isabel may have been a godmother of Isabella, Countess of Bedford, Edward and Philippa's second child.
Death and legacy.
Isabel died of the plague in 1361 at Leicester Castle. She was buried in Newark Abbey, Leicester. Her husband also died of the plague in March 1361.
With her husband, Isabel founded the College of the Annunciation of St Mary in the Newark, Leicester.
Through Blanche, Isabel was an ancestress of Englands's Royal House of Lancaster, with Henry IV of England being her grandson. Philippa of Lancaster, Queen of Portugal and the Algarve and Elizabeth of Lancaster, Duchess of Exeter were also her grandchildren.
References.
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