In his later years, Ivan's character, always stern, grew tyrannical. Apart from the reverses of the war, the change has been attributed to humiliations at the hands of the boyars during his childhood; a serious illness (1553) and resistance at that time to his efforts to secure the succession of his infant son; the death of his wife, Anastasia Romanov (1560), whom historians credit with exercising a moderating influence; and the defection to Poland of his favorite, Prince Andrew Kurbsky (1564). Suspecting conspiracies everywhere, he acted ruthlessly to consolidate his power. In 1565 he set aside an extensive personal domain, the oprichnina, under his direct control. He established a special corps ( oprichniki ), responsible to him alone, to whom he granted part of this domain at will. With the help of this corps, he diminished the political influence of the boyars and forcibly confiscated their lands in a reign of terror. Many boyars were executed or exiled.
Ivan formally abolished the oprichnina in 1572, although in effect it continued until 1575. Fits of rage alternated with periods of repentance and prayer; in one of his rages he killed (1581) his son and heir, Ivan. Although the exact number of his wives is uncertain, Ivan probably married seven times, ridding himself of unwanted wives by forcing them to take the veil or arranging for their murder. Despite his cruelty, he was a man of intelligence and learning. Printing was introduced into Russia during his reign. Two sons, Feodor I and Dmitri, survived the czar, but after his death his favorite, Boris Godunov, gained power.