Final answer:
Ivan III, also known as Ivan the Great, defeated the Mongols and marked the beginning of the Russian Empire by refusing to pay tribute to the Golden Horde and establishing Moscow as the center of power.
Step-by-step explanation:
The Slavic ruler who defeated the Mongols, marking the beginning of the Russian Empire, was Ivan III, also known as Ivan the Great. In the middle of the 15th century, Ivan III refused to pay tribute to the Golden Horde, the Mongol ruling authority at the time, and declared his kingdom as the new Rome. He tripled the size of his state, rebuilt the Kremlin in Moscow, and established Moscow as the center of what is now known as Russia. This resistance to Mongol rule and the fall of Constantinople led to the rise of the Russian Empire.