 Now one of Edward 's strongest supporters , he accompanied the king on a campaign to Scotland in 1314 , when several other nobles refused . He was killed at the Battle of Bannockburn on 24 June , under somewhat unclear circumstances . Gloucester was the most prominent of the casualties of the battle , which ended in a humiliating defeat for England . As he had no issue , his death marked the end of the prominent de Clare family . His estates were divided between his three sisters , one of whom was married to the king 's new favourite , Hugh Despenser the Younger . Despenser 's ruthless expansion of the de Clare lordship of Glamorgan in Wales led directly to the troubles of Edward II 's later reign , including a rebellion in the Welsh Marches , the defeat of the Earl of Lancaster at the Battle of Boroughbridge , and eventually the deposition of the king by Roger Mortimer and Queen Isabella in 1326 . 
