Simon de Montford, one of the Earl of Leicester, decided that he had a claim to the throne. He ruled essentially as the monarch, although he wasn't officially the monarch. A lot of the barons changed sides when they realised things weren't going their way. The battle at Yvesham was the end of Simon de Montford's reign. They actually chopped his body into pieces and sent the pieces around the country just in case any other Barons had ideas.