Accounts of the same event appearing in multiple sources are a relatively rare phenomenon for the Classical Middle Ages. Fortunately, the death of the Grand Prince of Rus’ Yaroslav Vsevolodovich at the Ordu of the Great Khan Güyük’s mother in September 1246 is recorded in the report of the envoy of the Apostolic See to the Mongols, John de Plano Carpini, and Russian medieval chronicles. This has led many historians to generally accept that he died from poison. However, some scholars challenge this interpretation, proposing that he may have succumbed to a food-borne illness and asserting that Russian sources are solely dependent on the news by the papal diplomat. They argue that a vassal in the Mongol Empire could only face punishment through a legitimate court. However, the study of inde-pendent sources on the history of the Mongol Empire demonstrates that the Chinggisids, using the scientific resources of the conquered peoples, had access to poisons and used them widely. Furthermore, reports of Yaroslav’s violent death surfaced in Northern Rus’ independently of Friar John’s narrative. Overall, a careful analysis of Friar John’s firsthand account reinforces the conclusion that the prince most likely died from poisoning.