The crossword solution for the term 'Terrible Russian Leader' is IVAN. IVAN was the first Tsar of Russia, ruling from 1547 to 1584. He was known for his brutal and oppressive rule, which included the massacre of thousands of people in Novgorod and the conquest of the Khanates of Kazan, Astrakhan, and Siberia. He also established the Oprichnina, a secret police force that terrorized the population. IVAN's reign was marked by political and social turmoil, and he is remembered as one of the most ruthless rulers in Russian history.