Final answer:
The Slavic ruler who defeated the Mongols and initiated the Russian Empire was Ivan III, also known as Ivan the Great. He expanded his territory significantly and his successor, Ivan IV, continued this expansion and became the first Tsar of all the Russias.
Step-by-step explanation:
The Slavic ruler who defeated the Mongols and played a pivotal role in marking the beginning of the Russian Empire was Ivan III, also known as Ivan the Great. Under Ivan III's leadership, who reigned from 1462 to 1505, the Grand Duchy of Moscow refused to pay tribute to the Golden Horde, signaling the decline of Mongol influence over the region. The Grand Duchy of Moscow, influenced by the fall of Constantinople and aspiring to become the 'Third Rome', saw significant territorial expansion and the rebuilding of the Kremlin during Ivan III's reign. Furthermore, his grandson, Ivan IV (Ivan the Terrible), who ruled from 1547 to 1584, was the first to officially claim the title of Tsar of all the Russias. Ivan IV continued the expansion by annexing the khanates of Kazan, Astrakhan, and Siberia and promoted colonization across Siberia.
While Mongol rule extended for a period in Russian lands with the establishment of the Golden Horde, by 1480, Russian princes had mustered enough strength to confront and ultimately weaken Mongol power. The final decline of the Mongols in Russia was marked by the conquest and destruction of the Mongol capital, Sarai, in 1556. Following this, the Golden Horde fractured into smaller states, with the Crimean Khanate enduring until its annexation by Russia in 1783.