Answer
Ögedei was the brother to whom Tolui swore allegiance as the Great Khan of the Mongol Empire. Tolui was the youngest son of Genghis Khan, and Ögedei was the third. Upon Genghis Khan's passing, Ögedei inherited the throne, and Tolui became one of his most loyal and trusted generals. Together, they led the Mongols in numerous successful military campaigns, expanding the empire to unprecedented heights.