Known as "Brother Number Two," Nuon Chea was the right-hand man and brother-in-law of notorious Khmer Rouge dictator Pol Pot and a key ideologist throughout the regime's reign of terror from 1975 to 1979.
Nuon Chea died in hospital, according to the Extraordinary Chambers in The Courts of Cambodia (ECCC). The United Nations-backed tribunal court was formed in 2006 to prosecute Khmer Rouge figures for alleged violations of international law during the Cambodian genocide.
In 2018, almost four decades after from the collapse of Pol Pot's tyrannical communist regime, the international tribunal ruled that the Khmer Rouge committed genocide. Nuon Chea, who was arrested in 2007, was found guilty of "of genocide, crimes against humanity and grave breaches of the Geneva Conventions of 1949." He was sentenced to life in prison.
He was already serving life sentence in Cambodia after being found guilty of crimes against humanity in 2014.
Nuon Chea was convicted alongside Khieu Samphan, or "Brother Number Four" -- they were two of the most senior leaders of the brutal Khmer Rouge regime to be prosecuted for the genocide.
The pair were also found guilty of murder, extermination, deportation, enslavement, imprisonment, torture, persecution on political, religious and racial grounds, and other inhumane acts.
About a quarter of Cambodia's population -- at least 1.7 million people -- are believed to have died during that period, from forced labor, starvation and execution.
During its time in power, the Khmer Rouge regime attempted to create a purely agrarian society through ruthless social engineering policies.
In 2001, Cambodia's National Assembly voted to create a court to try the crimes of those associated with the regime.
But for many, the few convictions that have followed have done little to heal wounds left by the Khmer Rouge.
"No action can assuage the anguish, sadness and regret that haunts the survivors to this day," former Khmer Rouge prisoner Youk Chhang wrote for CNN in 2014. "Over 35 years after the fall of the Khmer Rouge regime, we still see the effects from this period in almost every facet of Cambodian society."
Nuon Chea was born in 1926. After the collapse in 1979 of Democratic Kampuchea, as Cambodia was known, he remained a leading Khmer Rouge figure as the movement operated as a rebel guerrilla force in the country's west.
He surrendered in 1998, striking a deal with the government that allowed him to live as a free man near the Thai border until his arrest in 2007, according to the ECCC.
In his final statement to the court, Nuon Chea admitted he carried "moral responsibility" for events during the period, but also affirmed his innocence.