It is a large city within the borders of Khuraasaan, and it lies now in the North-East of Iran. The Tatars moved towards it after they had ruined the city of Marw entirely.

They besieged Nishapur for five days. Regardless of the reasonable number of the Muslim soldiers who gathered to defend it, the news of Marw had previously reached them, which caused terror to creep into their hearts, who, in turn, failed to resist the Tatars.

Thus, the Tatars entered the city and took out its inhabitants to the desert. Since the news reached the son of Genghis Khan that some of the inhabitants of Marw were saved from death, as the sword strikes they received were not fatal, and when the Tatars thought they died, they left them, he then commanded that all men of Nishapur should be killed and their heads cut off in order to be certain about their death.

Then, the accursed man captured all the Muslim women of the city in which they stayed for fifteen days, inspecting the houses in search for money and valuable things, after which they left Nishapur, which had turned into ruins.

https://en.islamstory.com/index.php/en/artical/20719/invasion-nishapur