Oleg of Novgorod/Kiev (??? - 912) (Helgi, in Old Norse) was a prominent prince of Kievan Rus' and a relative of the legendary founder of Novgorod, Rurik. As a ruler, he is credited with moving the capital from Novgorod to Kiev, thus paving the way for the rise of the Rus' empire. Additionally, he constructed numerous fortifications along the Dnieper and Dniester rivers, strengthening trade routes and establishing relations with the Byzantine Empire. 

In 882, Oleg conquered the nomadic peoples of the steppes and took control of the aforementioned Kiev. In 907, he launched a massive invasion of Constantinople with over 2,000 ships and 80,000 men, including Vikings. To avoid defeat, the Byzantine Empire offered Oleg a substantial tribute in 908 and exempted Russian merchants from customs duties, significantly increasing the prosperity of his kingdom like never before.

Much of Oleg's fame, beyond his military conquests, stems from his nickname, "the Prophet." According to the "Chronicle of Bygone Years", the Byzantines, seeking revenge for his siege, attempted to poison Oleg. However, displaying what were considered prophetic powers, he refused to drink the poisoned wine, saving himself. More likely, Oleg suspected the deception, as poisoning through contaminated drinks was a common practice during the Middle Ages to eliminate rulers.

In the "Chronicle of Novgorod" and the ballad "The Song of Wise Oleg", which both refer to him as a prophet (a Scandinavian term also used for a priest), it is said that Oleg was foretold he would die because of his horse. To avoid this fate, the Varangian prince immediately ordered the animal to be sent away. Years later, upon learning of the horse’s death, Oleg, believing the danger had passed, decided to see the animal's remains. As he bent down to touch the horse’s skull in a final gesture of affection, a snake emerged from it and bit him, leading to his death shortly after in 912, thus fulfilling the prophecy.

Despite his achievements as a ruler, Oleg's death led to uprisings among the subjugated populations, who reclaimed their autonomy, thereby destabilizing the Rus' river trade routes.