Answer
Louis IX, King of France from 1226 to 1270, died in Tunis on August 25, 1270, during the Eighth Crusade. The crusade was intended to capture Tunis, the capital of the Hafsid dynasty, which had recently converted to Islam. Louis IX was captured by the Hafsids and died of dysentery shortly thereafter. His death was a major setback for the crusade, which eventually failed.