Final answer:
Ivan IV, known as Ivan the Terrible, was the first Tsar of all the Russias and expanded the territory significantly. His moniker reflects his formidable reign.
Step-by-step explanation:
Ivan IV, commonly known as Ivan the Terrible, was the first ruler to officially adopt the title of Tsar of all the Russias, which is related to the term "Caesar." He reigned from 1547-1584 and is recognized for significant territorial expansions, notably the annexation of the Khanates of Kazan, Astrakhan, and Siberia. His rule also saw the colonization of Siberia, facilitated through the recruitment of Cossacks. His moniker, "the Terrible," comes from the Russian word "Grozny," which can be translated as "formidable" or "fearsome" and reflects both his autocratic rule and the legacy of his reign which was marked by both accomplishments and acts of severe oppression.