Ata-Malik Juvayni
According to "The Secret History of the Mongols", Tolui sacrificed himself in order to cure Ögödei from a very severe illness during a campaign in China. The shamans had determined that the root of Ögödei's illness were China's spirits of earth and water, who were upset that their subjects had been driven away and their land devastated. Offering land, animals, and people had only led to an aggravation of Ögödei's illness, but when they offered to sacrifice a family member, Ögödei got better immediately. Tolui volunteered and died directly after consuming a cursed drink. However, Ata-Malik Juvayni says he died from alcoholism.