Islamic Studies
Caliphate & Rulers: From the Rightly Guided Caliphs to the Ottoman Caliphate
Which Abbasid Caliph was killed when Baghdad fell to the Mongols?
Explanation
Al-Musta'sim Billah (640–656 AH) was the last Abbasid Caliph who was killed when Hulagu Khan destroyed Baghdad in 656 AH.
Reference: Islamic Studies - Caliphate & Rulers
, Page 13