THE MONGOL EMPIRE.

The Mongol Empire emerged from the unification of nomadic tribes in the Mongolia homeland under the leadership of Genghis Khan, who was proclaimed ruler of all Mongols in 1206.
The Keraites were one of the 5 dominant Turco-Mongol tribal confederations (Khanates) in the Altai-Sayan Region during the 12th century. They first enter into history as the ruling faction of the Zubu Confederacy, a large alliance of tribes that dominated Mongolia during the 11th and 12th centuries. Their originally territory corresponded to much of what is now Mongolia, along the Upper Onon and KerUlen Rivers and along the Tula. They often fought with the LiAo Dynasty of Northern China, which controlled much of Mongolia at the time. At the height of its power, the Keraites Khanate was organized along the same lines as the NaiMans and other powerful steppe tribes of the day.
They arrived in Europe with the Mongol Invasion led by Batu Khan and MonGke Khan. A portion were settled in Carpathian Galicia as a result of a hostage exchange treaty between Batu Khan and the Catholic Rus'Prince Daniel of Carpathian Galicia in 1246.
They had converted to the Church of the East (Nestorianism) in the early 11th century. This doctrine emphasized the disunion between the human and divine natures of Jesus, and was advanced by Nestorius (386-450) Patriarch of Constantinopla from 428-431. His teachings were condemned as heretical leading to Nestorian Schism, in which churches supporting Nestorius broke with the rest of the Christian Church.
The Keraites are first noted in Syriac Church records which mention them being absorbed into it around 1000CE by the Metropolitan Province of the Church between the 5th and 11th centuries. One Syrian diocese in Khorasan, in Iran, existed by the begining of the 5th century, though it was not assigned to a Metropolitan Province in 410. After establishing 5 Metropolitan Provinces in Mesopotamia, Canon XXI of the Synod of Isaac provided that "the Bishops of the more remote dioceses of Fars, of the Islands, of Beth Madaye, of Beth Raziqaye, and of the country of AbrAshAhr must accept the definition established in this Council at a later date." By implication, AbrAsh Ahr (NishApur) already had a bishop in this period.
The growing importance of the Merv Region for the "Church of the East" is attested by the appearance of several more Christian Centres during the late 5th and 6th century. In 893 Eliya of Damascus listed both Merv and Herat as Metropolitan Provinces.
Bt the 11th century East Syrian Cristianity was in decline. The surviving urban Christian communities in Khorasan, Iran, suffered a heavy blow at the start of the 13th century, when the cities of Merv, NishApur, and Herat were stormed by Genghis Khan in 1220. Their inhabitants were massacred.
They were defeated by Genghis Khan in 1203 and became influential in the rise of the Mongol Empire, and were gradually absorbed into the succeeding Turco-Mongolian Khanates during the 13th century.
Mongol legend traced the Clan back to 8 brothers with usually dark faces whose confederation they founded was called Kerait ("Black, Swarthy"). Kerait was the name of the leading brother's clan, while the others are recorded as JirKin, KonKant, SaKait, TumAut, AlBat. An incorporation of sub-clans may have led to the Turco-Mongol amalgamation from early time.
Kan is a title equal to the status of emperor. It is also translated as Khan of Khans, equivalent to King of Kings. Only Genghis Khan and his ruling descendants are called Khagan, while other rulers are referred to as Khan.

The Mongol Empire began to politically split with the Toluid Civil War in 1260-1264 and the death of Kublai Khan in 1294.