Simon de Montfort
From Ex illis wiki
Earl of Leicester (1239–1265)
Simon de Montfort is a troublesome English lord who tries to establish England’s first elected parliament. He leads opposition to King Henry III in 1258, and urges the king to agree to the Provisions of Oxford, a document that would give expanded rights to English barons. Henry revokes his assent to the Provisions in 1261, an act which incites Montfort to go into self-imposed exile.
He returns in 1263 to lead a real revolt against the Crown. Supported by several barons, Montfort engages in a war against the King in order to force him to restore the Provisions of Oxford. He wins the first battle against Henry in 1264 at Lewes, where both the King and Prince Edward are taken prisoners. He then sets up a government and calls a Parliament to order, in 1265, the first elected assembly of representatives. But the barons do not appreciate their powerless role in this assembly, and some rebel against Montfort. They support Prince Edward, who escapes captivity, and help the royal troops fight the Earl of Leicester. The two armies clash at the Battle of Evesham in 1265. Prince Edward carries the day; Montfort is killed on the battlefield.