I’ve never been able to identify the polity itself, but I’ve heard a story about how during the of time of the Mongols, a polity basically killed off one of the Mongol messengers and the consequences of this was the entire eradication of that one nation. I’m pretty sure it was located somewhere around South-West Pakistan.
Do you mean the Khwarizmian Empire? The Khwarizmians were a Turkic dynasty that ruled Persia and Central Asia (basically some of the modern -stan countries - not Pakistan, but Afghanistan, Tajikistan, Uzbekistan). The original ruler was the governor of the central Asian province of Khwarezm (Chorasan) in the Seljuk Empire in the 11th century, but they began to rule independently in the 12th century, and actually took over most of the Seljuk Empire when the Seljuks basically collapsed due to civil war.
They ruled from Samarkand in modern Uzbekistan. In 1218 a Mongol ambassador arrived, but the Khwarizmians killed him for some reason. The Mongols responded by utterly destroying the empire in 1220-21. After that, many of the surviving Khwarizmians wandered around the Middle East serving as mercenaries, and they eventually ended up allying with the Ayyubid sultan of Egypt against the other Ayyubids in Syria and the crusader states. (I answered a question recently about that period.)
There isn't very much written about the Khwarizmians in English, but here are some sources:
Rene Grousset, The Empire of the Steppes, trans. Naomi Walford (Rutgers University Press, 1970).
David Morgan, The Mongols, 2nd ed. (Blackwell, 2007)
Peter Jackson, The Mongols and the West (Routledge, 2005)
A.C.S. Peacock, The Great Seljuk Empire (Edinburgh University Press, 2015)