That immense Kara Kitai, or Black Cathay, or Black China was added to the Mongol dominions which now were conterminous with the Kwaresmian Empire. This Empire, begun on Seljuk ruins, was increased soon by other lands, and in 1219 it extended from the Syr Darya or Yaxartes to the Indus, and from Kurdistan to the great roof of the world, those immense Pamir highlands. The sovereign at the opening of the thirteenth century was Alai ud din Mohammed, great-great-grandson of a Turk slave named Nush Tegin. The master of this slave was a freedman of Melik Shah the Seljuk Sultan, and this freedman transferred Nush Tegin to his sovereign. The slave became cupbearer to Melik Shah, and prefect of Khwaresm at the same time by virtue of his office. In Mohammedan history cases of Turkish slaves seizing sovereignty are frequent. Turkish captives in Persia were highly esteemed and appeared there in multitudes. Throughout the vast regions north and east of the Caspian various Turk tribes fought unceasingly; each seized the children of an enemy whenever the chance came, and sold them in the slave marts. These children, reared in the faith of Mohammed, were trained to arms for the greater part, and became trusted body-guards of princes. They served also as household officials, or managers. Those of them who earned favor gained freedom most frequently, and next the highest places at courts, and in armies. A lucky man might be made governor, and when fortune helped well enough he made himself sovereign. Turkish slaves grew all-powerful in Moslem lands, till those lands were invaded at last by Turk warriors. Persia, lowered much by Arab conquest, recovered under Bagdad rule in some slight degree, till the eleventh century saw it conquered again by Turk nomads from those immense steppes north and east of the Caspian. Under the descendants of Seljuk these fierce sons of wild herdsmen pushed their way on to the Propontis and to Palestine; camped in Persia, and in lands lying west of it. These self-seeking, merciless adventurers brought torture, oppression, and brigandage to all people equally, till at last intestine wars and social chaos put an end to Seljuk rule toward the close of the twelfth century. Kutb ud din Mohammed, son of the manumitted slave, Nush Tejin, and also his successor, won the title of Kwaresmian Shah, a title used before the Arab conquest. Atsiz, son of Kutb ud din, raised arms repeatedly against Sindjar, the son of Melik Shah, and was forced to render tribute to the Gurkhan. When Sindjar died (1157) Il Arslan, son of Atsiz, seized West Khorassan; his son, Tukush, took Persian Irak from Togrul, who fell in battle. By the death of Togrul and Sindjar, both Persian Seljuk lines became extinct.