A demon core was part of early nuclear bomb research. The type of reaction they were studying would emit blue light and a ton of radiation.
https://en.wikipedia.org/wiki/Demon_core
It’s been a meme to imagine the demon core as an available weapon in medieval fantasy, since it looks somewhat like a mace.
https://newrepublic.com/series/49/microsoft-three-mile-island-ai-nuclear
Nope. Very real.