“King Alfonso of Castile, who was besieging the Muslim stronghold, was urged to flee to safety. However, the king, who had lost a future daughter-in-law to the plague two years earlier – Princess Joan of England – insisted on remaining with his army. On March 26, 1350, a Good Friday, Alfonso became the only reigning European monarch to die of the pestilence.”

“King Alfonso of Castile, who was besieging the Muslim stronghold, was urged to flee to safety. However, the king, who had lost a future daughter-in-law to the plague two years earlier – Princess Joan of England – insisted on remaining with his army. On March 26, 1350, a Good Friday, Alfonso became the only reigning European monarch to die of the pestilence.”

— John Kelly