From Wikipedia: Richard of Conisburgh , 3rd Earl of Cambridge (c. 1375 – 5 August 1415) was the younger son of Edmund of Langley, 1st Duke of York and Isabella of Castile.
His paternal grandparents were Edward III of Windsor, King of England and Philippa of Hainault. His maternal grandparents were Peter of Castile and MarÃa de Padilla. He was born at Conisburgh Castle in Yorkshire, and was confirmed in the Earldom of Cambridge, which had been resigned by his brother, in 1414.
Marriage
In about 1406, he married his cousin, Anne Mortimer, also a descendant of Edward III (his great-great-granddaughter), through his son Lionel of Antwerp. A papal dispensation was dated for 28 May 1406, making it most likely that the marriage took place in May or June. It was through her that the Yorkist faction in the Wars of the Roses claimed the throne. Their marriage produced a daughter, Isabel, and a son, Richard Plantagenet, 3rd Duke of York. The latter eventually laid claim to the throne, starting the Wars of the Roses. It is believed that Anne died giving birth to Richard. Following Anne’s death, Cambridge married Matilda Clifford.
Death
He was discovered to be one of the fomentors of the Southampton Plot against King Henry V immediately prior to departure on the French campaign. (His elder brother, Edward of Norwich, 2nd Duke of York, would die at the Battle of Agincourt, less than three months later.) He was stripped of all his titles and estates and was executed on 5 August 1415 at Southampton Green, Hampshire, England; before the fleet set sail on 11 August 1415.
Coat of arms 
Richard bore his father’s arms (those of the kingdom, differenced by a label argent of three points, each bearing three torteaux gules), differenced by a bordure Leon