Roger was originally on the side of Simon, but changed his mind when he heard of an alliance Simon had made.
Simon decided to make an ally of Llywelyn ap Gruffudd, the Prince of Gwynedd. Wales was England's enemy, and the followers of Simon started to turn on him when he and Llywelyn made peace. The Welsh Marcher Lords, English nobles guarding castles along the Welsh border, were friendly with Henry's eldest son Edward, and rallied around Edward and the royalist cause.
Prince Edward's forces defeated several of Montfort's allies at the Warwickshire market town of Kenilworth, even though the rebels had been bolstered by Welsh forces. Simon was moving across the Severn to join his son's army, and saw an army approaching flying his son's banners. Edward had seized the banners at Kenilworth and was using them to fool the rebels.
They encountered each other at the Battle of Evesham when Simon—who was hemmed on three sides by the River Avon—realized too late that he had been fooled and attempted a foolish uphill charge against a superior force. Edward had appointed a dozen men to avoid direct fighting in the encounter and search specifically for Simon. Having been located, Simon was stabbed in the neck by Roger Mortimer, 1st Baron Mortimer. The royalists attacked the corpse, cutting off his head, ripping out his intestines, cutting off his testicles and hanging them on his nose, etc. His hands and feet were cut off and sent to various parts of England as a sign of the consequences of treason. (The illustration is of Simon's body being mutilated on the battlefield.)
Fighting continued, but eventually the royalists won. There are two follow-ups to offer: the Dictum of Kenilworth between Henry and the rebels, and the story of Simon's niece by marriage who was henry's daughter, Margaret of England. We'll cover Kenilworth tom arrow and Margaret the next day.

















