For some context to understand the awfulness of his Empire:

On The Tatars - Ibn al-Athir

For some years I continued averse from mentioning this event, deeming it so horrible that I shrank from recording it and ever withdrawing one foot as I advanced the other. To whom, indeed, can it be easy to write the announcement of the death-blow of Islam and the Muslims, or who is he on whom the remembrance thereof can weigh lightly? O would that my mother had not born me or that I had died and become a forgotten thing ere this befell! Yet, withal a number of my friends urged me to set it down in writing, and I hesitated long, but at last came to the conclusion that to omit this matter could serve no useful purpose.

I say, therefore, that this thing involves the description of the greatest catastrophe and the most dire calamity (of the like of which days and nights are innocent) which befell all men generally, and the Muslims in particular; so that, should one say that the world, since God Almighty created Adam until now, has not been afflicted with the like thereof, he would but speak the truth. For indeed history does not contain anything which approaches or comes near unto it. For of the most grievous calamities recorded was what Nebuchadnezzar inflicted on the children of Israel by his slaughter of them and his destruction of Jerusalem; and what was Jerusalem in comparison to the countries which these accursed miscreants destroyed, each city of which was double the size of Jerusalem? Or what were the children of Israel compared to those whom these slew? For verily those whom they massacred in a single city exceeded all the children of Israel. Nay, it is unlikely that mankind will see the like of this calamity, until the world comes to an end and perishes, except the final outbreak of Gog and Magog.

For even Antichrist will spare such as follow him, though he destroy those who oppose him, but these Tatars spared none, slaying women and men and children, ripping open pregnant women and killing unborn babes. Verily to God do we belong, and unto Him do we return, and there is no strength and no power save in God, the High, the Almighty, in face of this catastrophe, whereof the sparks flew far and wide, and the hurt was universal; and which passed over the lands like clouds driven by the wind. For these were a people who emerged from the confines of China, and attacked the cities of Turkestan, like Kashghar and Balasaghun, and thence advanced on the cities of Transoxiana, such as Samarqand, Bukhara and the like, taking possession of them, and treating their inhabitants in such wise as we shall mention; and of them one division then passed on into Khurasan, until they had made an end of taking possession, and destroying, and slaying, and plundering, and thence passing on to Ray, Hamadan and the Highlands, and the cities contained therein, even to the limits of Iraq, whence they marched on the towns of Adharbayjan and Arraniyya, destroying them and slaying most of their inhabitants, of whom none escaped save a small remnant; and all this in less than a year; this is a thing whereof the like has not been heard. And when they had finished with Adharbayjan and Arraniyya, they passed on to Darband-i-Shirwan, and occupied its cities, none of which escaped save the fortress wherein was their King; wherefore they passed by it to the countries of the Lan and the Lakiz and the various nationalities which dwell in that region, and plundered, slew, and destroyed them to the full. And thence they made their way to the lands of Qipchaq, who are the most numerous of the Turks, and slew all such as withstood them, while the survivors fled to the fords and mountain-tops, and abandoned their country, which these Tatars overran. All this they did in the briefest space of time, remaining only for so long as their march required and no more.

Another division, distinct from that mentioned above, marched on Ghazna and its dependencies, and those parts of India, Sistan and Kirman which border thereon, and wrought therein deeds like unto the other, nay, yet more grievous. Now this is a thing the like of which ear has not heard; for Alexander, concerning whom historians agree that he conquered the world, did not do so with such swiftness, but only in the space of about ten years; neither did he slay, but was satisfied that men should be subject to him. But these Tatars conquered most of the habitable globe, and the best, the most flourishing and most populous part thereof, and that whereof the inhabitants were the most advanced in character and conduct, in about a year; nor did any country escape their devastations which did not fearfully expect them and dread their arrival.

Moreover they need no commissariat, nor the conveyance of supplies, for they have with them sheep, cows, horses, and the like quadrupeds, the flesh of which they eat, naught else. As for their beasts which they ride, these dig into the earth with their hoofs and eat the roots of plants, knowing naught of barley. And so, when they alight anywhere, they have need of nothing from without. As for their religion, they worship the sun when it rises, and regard nothing as unlawful, for they eat all beasts, even dogs, pigs, and the like; nor do they recognise the marriage-tie, for several men are in marital relations with one woman, and if a child is born, it knows not who is its father.

Therefore Islam and the Muslims have been afflicted during this period with calamities wherewith no people hath been visited. These Tatars (may God confound them!) came from the East, and wrought deeds which horrify all who hear of them, and which you shall, please God, see set forth in full detail in their proper connection. And of these was the invasion of Syria by the Franks (may God curse them!) out of the West, and their attack on Egypt, and occupation of the port of Damietta therein, so that Egypt and Syria were like to be conquered by them, but for the grace of God and the help which He vouchsafed us against them, as we have mentioned under the year 614 (A.D. 1217-18). Of these, moreover, was that the sword was drawn between those who escaped from these two foes, and strife was rampant, as we have also mentioned: and verily unto God do we belong and unto Him do we return! We ask God to vouchsafe victory to Islam and the Muslims, for there is none other to aid, help, or defend the True Faith. But if God intends evil to any people, naught can avert it, nor have they any ruler save Him. As for these Tatars, their achievements were only rendered possible by the absence of any effective obstacle; and the cause of this absence was that Muhammad Khwarazmshah had overrun the lands, slaying and destroying their Kings, so that he remained alone ruling over all these countries; wherefore, when he was defeated by the Tatars, none was left in the lands to check those or protect these, that so God might accomplish a thing which was to be done.

It is now time for us to describe how they first burst forth into the lands. Stories have been related to me, which the hearer can scarcely credit, as to the terror of the Tatars, which God Almighty cast into men's hearts; so that it is said that a single one of them would enter a village or a quarter wherein were many people, and would continue to slay them one after another, none daring to stretch forth his hand against this horseman. And I have heard that one of them took a man captive, but had not with him any weapon wherewith to kill him; and he said to his prisoner, "Lay your head on the ground and do not move," and he did so, and the Tatar went and fetched his sword and slew him therewith. Another man related to me as follows: "I was going," said he, "with seventeen others along a road, and there met us a Tatar horseman, and bade us bind one another's arms. My companions began to do as he bade them, but I said to them, "He is but one man; wherefore, then, should we not kill him and flee?' They replied, 'We are afraid.' I said, 'This man intends to kill you immediately; let us therefore rather kill him, that perhaps God may deliver us.' But I swear by God that not one of them dared to do this, so I took a knife and slew him, and we fled and escaped.' And such occurrences were many.