After a siege of weeks and the collapse of its walls' defence, Baghdad surrendered to Hülegü's Mongol army on 4 Ṣafar 656. What followed filled the sources on every side: a sack lasting days, the libraries emptied into the Tigris until — in the famous image — the river ran black with ink, and the killing counted by the hundreds of thousands.
The caliph al-Mustaʿṣim was executed some ten days later, rolled in carpet and trampled so that no royal blood would touch the ground — the Mongols' own scruple. Five centuries of ʿAbbāsid Baghdad ended; the shock registered from Delhi to Cairo, where a shadow caliphate was soon installed under Mamluk protection.
The entry date is carried precisely; the sack and the caliph's death spread over the following days.