His influence over Park Geun-hye, daughter of South Korean dictator Park Chung-hee, began in the 1970s and reportedly endured for decades — extending, through his daughter Choi Soon-sil, into the years of Park Geun-hye's presidency itself. The relationship became central to one of South Korea's most significant political scandals, raising questions about how deeply a single private individual had shaped the decisions of a sitting head of state.

His reign divides sharply into two phases: an early period of genuine institutional reform and military expansion, and a later descent into paranoid repression that gave him his enduring epithet. The oprichnina — a state within a state staffed by personal loyalists — became the instrument of mass executions, forced relocations, and the destruction of the boyar class. The 1570 sack of Novgorod, carried out on his orders against his own subjects, remains one of the most devastating episodes of internal violence in Russian history. He consolidated and expanded the Russian state while simultaneously terrorizing it, a contradiction that has made him one of the most studied rulers of the early modern period.