Demon Core
Unlike the mana cores used by humans, which are usually made many years after their birth and are mainly used for gaining and using mana,.
A demon core is part of their natural anatomy and has been present inside them since birth.
It's the most important body part of their body, as not only does it let them use miasma, but it also makes them immortal for as long as their core is safe.
This was the reason Kaust could come back to life even after William had decapitated him.
This also means that unless a demon is killed or starves, they can't die since demons don't age.
Another thing about a demon core was that a demon couldn't destroy his own core; usually, this would not be relevant as no demon would ever willingly try to end their own life.
However, this also means that Kaust was no longer in control of his own life and that his fate was completely in the hands of the being inhibiting Williams body.
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