John II of France

John II became King of France in 1350 as the Black Death tore through the kingdom, killing up to half the population, and English forces pressed hard in the Hundred Years' War. Six years into his reign he was captured at the Battle of Poitiers and held prisoner in London while his son Charles ruled as regent and put down revolts at home. Charles eventually signed the Treaty of Brétigny in 1360, surrendering vast territories and pledging an enormous ransom to free his father; John was released in exchange for hostages including his son Louis I, Duke of Anjou, and returned to France to raise the ransom funds. He created the franc to stabilise the currency and tried to dispatch the mercenary free companies ravaging the countryside on a crusade, but the plan collapsed when Pope Innocent VI died. When word reached him that Louis had escaped English custody, John turned around and went back to London of his own will, and he died there.