Complex sentence: The Kereit (Mongolian: Хэрэйд;) tribe was one of the five major tribal confederations (khanlig) in Mongolian plateau in the 12th century, and dominant in the area and, as allies of Genghis Khan, influential in the rise of the Mongol Empire.
Split into simpler sentences: The Kereit (Mongolian: Хэрэйд;) tribe was the most dominant of the five major tribal confederations (khanlig) of the Mongolian plateau during the 12th century. As allies of Genghis Khan, they were influential in the rise of the Mongol Empire.