Genghis Khan invaded the eastern part of 'Ala' ad-Din's empire in 1219. Avoiding a battle, 'Ala' ad-Din retreated to a small island in the Caspian Sea, where he died in 1220. Soon after 'Ala' ad-Din's death, his energetic son Jalal ad-Din Mingburnu rallied the Afghan highlanders at Parwan (modern Jabal os Saraj), near Kabul, and inflicted a crushing defeat on the Mongols under Kutikonian. Genghis Khan, who was then at Herat, hastened to avenge the defeat and laid siege to Bamian. There Mutugen, the Khan's grandson, was killed, an event so infuriating to Genghis Khan that when he captured the citadel he ordered that no living being be spared. Bamian was utterly destroyed. Advancing on Ghazna, Genghis won a great victory over Jalal ad-Din, who then fell back toward the Indus (1221), where he made a final but unsuccessful stand.