The term 'TERRIBLE RULER' can be summarized by two crossword solutions: IVAN (4 letters) and Ivaniv (6 letters). IVAN is likely referring to IVAN the TERRIBLE, the Grand Prince of Moscow from 1533-1584 and the first Tsar of All Russia from 1547-1584. He was known for his cruel and oppressive rule, earning him the title of 'TERRIBLE'. Ivaniv is likely referring to IVAN IV Vasilyevich, also known as IVAN the TERRIBLE, who was the Grand Prince of Moscow from 1533-1584 and the first Tsar of All Russia from 1547-1584. He was known for his tyrannical rule and is remembered as one of the most notorious rulers in Russian history.