 Charles ultimately died at the Battle of Auray in 1364, fighting the forces of the younger John.In the Treaty of Guérande, which ended the War of the Breton Succession, the Duchy of Brittany was made hereditary in the males of the Montfort line, at the termination of which it was to pass to the males of Blois-Penthièvre.In 1420 the Penthièvre family imprisoned the duke in a failed attempt to regain Brittany.