“The Demon Core”, a mass of plutonium that was responsible for the death of two scientists on two separate occasions.
It only got that name due to being involved in the two criticality accidents that killed two people, and what happened is fairly simple–people didn’t treat a highly dangerous material with the respect it was due.
In one of the incidents, for example, they were using a hand-held screwdriver to hold the two halves of the core apart to keep it under the critical mass while experimenting on it, and the screwdriver slipped and momentarily allowed the core to go critical–it was only the researcher reaching in and pulling the two halves of the core apart by hand that prevented a much worse disaster.
However, him being closest to the thing meant he received a fatal dose of hard radiation.
As to what happens when a lump of radioactive material goes critical–material undergoing radioactive decay emits smaller particles as the atoms break up, usually neutrons and alpha particles.
Some of these go on to hit other atomic nuclei and cause those to break up in turn, but this is a fairly uncommon event because atomic nuclei are very small and the distances between them comparatively large.
However, once you have a critical mass of the material enough of these collisions occur to form a self-sustaining chain reaction–e.g. when one atom breaks up, the pieces flung out will hit other atoms and cause those to break up, which flings out more pieces and breaks up more atoms, etc.
This massively increases the energy and radiation output of the material and, when set up properly in an atomic bomb, causes a huge explosion.
