The last Abbasid caliph of Baghdad, whose reign ended in 1258 with the Mongol sack of the city — the fall of the five-century Abbasid caliphate.
Life
Al-Mustasim (reigned 1242–1258) was the thirty-seventh and final Abbasid caliph in Baghdad. Irresolute before the Mongol advance, he refused Hulagu Khan's demands; the Mongols besieged and sacked Baghdad in 1258 with immense slaughter and the destruction of its libraries, and executed the caliph — ending the Abbasid caliphate of Baghdad in one of the great catastrophes of Islamic history.