Write a short paragraph that identifies the Russian leader who came to power at the age of three and earned the nickname "the Terrible."

Historic sources present disparate accounts of Ivan's complex personality: he was described as intelligent and devout, yet given to rages and prone to episodic outbreaks of mental instability that increased with his age, affecting his reign. In one such outburst, he killed his groomed and chosen heir Ivan Ivanovich. This left the Tsardom to be passed to Ivan's younger son, the weak and intellectually disabled Feodor Ivanovich.