Answer
Yermak Timofeyevich, a Cossack leader, served under Ivan IV, commonly known as Ivan the Terrible, from the mid-1570s until his death in 1584. Yermak is primarily remembered for his conquest of the Siberian Khanate, which marked the beginning of Russia's eastward expansion. Under Ivan's reign, Yermak received support and encouragement for his military campaigns, expanding the Russian Empire's influence and territories.