I highly recommend you take the time this fascinating New Yorker piece on the Mongols and the sacking of Baghdad. An excerpt:
On January 29, 1258, Hulagu’s forces took up a position on the eastern outskirts of Baghdad and began a bombardment. Soon they had breached the outer wall. The caliph, who had been advised against escaping by his vizier, offered to negotiate. Hulagu, with the city practically in his hands, refused. The upshot was that the caliph and his retinue came out of the city, the remainder of his army followed, they laid down their arms, and the Mongols killed almost everybody. Hulagu told Baghdad’s Christians to stay in a church, which he put off-limits to his soldiers. Then, for a period of seven days, the Mongols sacked the city, killing (depending on the source) two hundred thousand, or eight hundred thousand, or more than a million. The Mongols’ Georgian Christian allies were said to have particularly distinguished themselves in slaughter. Plunderers threw away their swords and filled their scabbards with gold. Silver and jewels and gold piled up in great heaps around Hulagu’s tent. Fire consumed the caliph’s palace, and the smoke from its beams of aloe wood, sandalwood, and ebony filled the air with fragrance for a distance of a hundred li. (A li equalled five hundred bow lengths—a hundred li was maybe thirty miles.) So many books from Baghdad’s libraries were flung into the Tigris that a horse could walk across on them. The river ran black with scholars’ ink and red with the blood of martyrs.
The stories of what Hulagu did to the caliph vary. One says that Hulagu toyed with him a while, dining with him and discussing theology and pretending to be his guest. A famous account describes how Hulagu imprisoned the caliph in a roomful of treasure and brought him gold on a tray instead of food. The caliph protested that he could not eat gold, and Hulagu asked him why he hadn’t used his money to strengthen his army and defend against the Mongols. The caliph said, “That was the will of God.” Hulagu replied, “What will happen to you is the will of God, also,” leaving him among the treasure to starve.