What "his" refers to?

I want to know what the underlined "his" refers to in this paragraph. Is it Ivan the Great or the Monk? Thanks in advance.

Through conquest, diplomacy, and his rejection of Moscow's two-hundred year loyalty to the Mongols, Ivan the Great made Russia a European power. When he assumed the throne, his realm consisted of just fifteen thousand square miles, less than half the size of present day Indiana State of the United States. With his son Vasily, he nearly tripled Russian territory. Ivan the Great dreamed of making Russia the Third Rome, cherishing a monk's prophecy that while "the two Romes have fallen, the third does endure. Your Christian Empire shall last forever." To fulfill "his" vision, he claimed to be a direct descendant of the Roman emperor Caesar Augustus, and he married the niece of the last emperor of Byzantium (the second Rome).