It's the core of a nuclear bomb. It would have been the third to be dropped if Japan hadn't surrendered. So now they had this 14 lbs sphere of plutonium just on the verge of going critical and didn't know what to do with it except science. It's often depicted with these two hemispheres that surround the core. Those are neutron reflectors made of beryllium. If those close together completely, it'll make the core go critical by reflecting enough neutrons back into the core and cause nuclear reactions. (It won't explode though. It'll just get really spicy with you seeing blue lights form in your eyes.)
So it started with a scientist accidentally dropping a brick of this reflector stuff on the core. It briefly went supercritical and dosed him with enough radiation to kill.
Next came a guy named Slotin. He is a scientist best described by the terms: "Fucked around and found out." The experiment was to determine how close they could get this thing to go critical by varying the gap between the aforementioned hemispheres. They told him to use the specially designed shims which prevented the thing from completely closing and giving everyone around it a lethal dose of radiation.
He used a flat head screwdriver and no shims.
He kept doing this unapproved procedure for quite a bit. Murphy's law suddenly went "Ayo grip check," and the driver slipped.
The demon core is directly responsible for 2 deaths and is directly linked to the cancer that developed in 4 more.
tl/dr: Don't fuck around with nuclear material, especially those on the verge of becoming a spicy radioactive rock.
