The Plantagenet Roll of the Blood Royal: Essex Volume. Being a Complete Table of All the Descendants Now Living of Edward III, King of England: The Isabell of Essex Volume, Containing the Descendants of Isabel (Plantagenet) Countess of Essex and Eu. The Marquis of Ruvigny and Raineval. The Lady Isabel was the only daughter of Richard, Earl of Cambridge, who was beheaded at Southampton, 05 August 1415. Her mother was Lady Anne Mortimer, sister and sole heir of Edmund, 5th Earl of March.