I left out the suffering details. Honestly all the radiation related deaths are horrific. That's what I learned from the Plainly Difficult YouTube channel.
The "demon core" was a subcritical mass of plutonium-239 that was used in two criticality accidents at the Los Alamos laboratory in 1945 and 1946, resulting in the acute radiation poisoning and subsequent death of two scientists. It was called the "demon core" because of these incidents.
But burn it all down makes for great lyrics.