Question: Yelü Chucai was a retainer of which founder of the Mongol Empire?

<P0-S0> Mongol conquests of Kingdom of Georgia, which at that time consisted of Georgia proper, Armenia, and much of the Caucasus, involved multiple invasions and large-scale raids throughout the 13th century. <P0-S1>  The Mongol Empire first appeared in the Caucasus in 1220 as generals Subutai and Jebe pursued Muhammad II of Khwarezm during the destruction of the Khwarezmian Empire. <P0-S2>  After a series of raids in which they defeated the Georgian and Armenian armies (Battle of Caucasus Mountain), Subutai and Jebe continued north to invade Kievan Rus'. <P0-S3>  After his empire was destroyed, Khwarazm ruler Jalal ad-Din Mingburnu, son of Muhammed II, battled both the Mongols and the Georgians before moving on to challenge the Seljuks in Anatolia. <P0-S4>  A full-scale Mongol conquest of the Caucasus and eastern Anatolia began in 1236, in which the Kingdom of Georgia, the Sultanate of Rum, and the Empire of Trebizond were subjugated, the Armenian Kingdom of Cilicia and other Crusader states voluntarily accepted Mongol vassalage, and the Assassins were eliminated. <P0-S5>  The Mongols also invaded Dzurdzuketia, modern-day Chechnya, but faced continual resistance in that area. <P0-S6>  After the death of Möngke Khan in 1259, the Mongol Empire descended into civil war and Berke of the Golden Horde and Hulagu of the Ilkhanate repeatedly invaded each other in the Caucasus until the ascension of Kublai Khan in 1264. <SEP> <P1-S0> The list of Khitan inscriptions comprises a list of the corpus of known inscriptions written in the Khitan large script and the Khitan small script. <P1-S1>  These two scripts were used by the Khitan people in northern China during the 10th through 12th centuries for writing the extinct Khitan language. <P1-S2>  The Khitan language was in use during the Liao Dynasty (907–1125), the Kara-Khitan Khanate (1124–1218) and the Jin dynasty (1115–1234), but the last recorded Khitan speaker, Yelü Chucai, died in 1243, and the language probably became extinct soon afterwards. <SEP> <P2-S0>  He came to power by uniting many of the nomadic tribes of Northeast Asia. <P2-S1>  After founding the Empire and being proclaimed "Genghis Khan", he launched the Mongol invasions that conquered most of Eurasia. <P2-S2>  Campaigns initiated in his lifetime include those against the Qara Khitai, Caucasus, and Khwarazmian, Western Xia and Jin dynasties. <P2-S3> Genghis Khan (c. 1162 August 18, 1227), born Temüjin, was the Great Khan and founder of the Mongol Empire, which became the largest contiguous empire in history after his death. <P2-S4>  These campaigns were often accompanied by large-scale massacres of the civilian populations – especially in the Khwarazmian and Western Xia controlled lands. <P2-S5>  By the end of his life, the Mongol Empire occupied a substantial portion of Central Asia and China. <SEP> <P3-S0>  He was the first of Genghis Khan's retainers to formulate policy during the Mongol conquests, and he also introduced many administrative reforms in North China during the reign of Genghis Khan and his successor Ögedei. <P3-S1> Yelü Chucai (Yeh-lu Chu-tsai; ; Mongolian: Urtu Saqal, 吾圖撒合里, "long beard"; the components of his name also variously spelt Yeh-Lu, Ye Liu, Yeliu, Chutsai, Ch'u-Ts'ai, etc.) (July 24, 1190 - June 20, 1244) was a statesman of Khitan ethnicity with royal family lineage to the Liao Dynasty, who became a vigorous adviser and administrator of the early Mongol Empire in the Confucian tradition.