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.
🎶 It’s gonna take a lot to drag me away from you 🎶
🎵 There’s nothin’ that a hundred men or more could ever do 🎵
🎶 I bless the rains in South America 🎶