Excerpts from Books and Wikipedia
"He opened his country up to trade with the West, introduced printing, set up his own secret police, reduced the power of the noble Boyars, and expanded his realm into Siberia and Tartary. But he was also--especially late in his reign--cruel, dissolute, and unpredictable. . . . Ivan became increasingly paranoid after a serious illness in 1553. Following the death of his beloved first wife, Anastasia, in 1560, his behavior . . . began to suggest serious mental instability. . . . An analysis of Ivan's remains in the 1960s found high levels of mercury--suggesting that he had been poisoned. Alternatively, he may have taken treatments containing mercury for syphilis, a condition that may have contributed to his mental instability." [Furtado: 1001 Days] "He imprisoned and executed boyars (nobles) who opposed him, and confiscated their estates. Vast numbers of oppressed people fled to newly acquired lands along the Don and Volga rivers, where they could be free of Ivan's arbitrary rule. These refugees were known as Cossacks." [DK Timelines, p. 283] "The Tartar khanate of the Crimea remained a powerful foe; its troops sacked Moscow in 1571, and it remained independent until the late eighteenth century." [Kennedy, Great Powers, p. 16]