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So, I don't exactly have the capability to actually write a fanfic like that so here is the idea and anyone at all can write it. Just please tell me so I can read too.
So in Demon Slayer, marechi blood is described as extremely rare, intoxicating, and vastly more nourishing to demons than normal human blood. One marechi can be worth dozens to hundreds of ordinary humans, depending on the purity and potency of their blood.
Now, if we take Cale Henituse (Kim Rok Soo) and drop him into that universe:
Vitality of the Heart:
His "indestructible heart" and immense life force would be practically off the charts in terms of Demon Slayer standards.
It’s not just rare marechi, it would be like the ultimate delicacy: perfectly rich, endlessly regenerative, almost impossible to consume fully.
Equivalent in Human Lives:
Normal marechi in the series is said to be equivalent to 50–100 humans worth of blood for a demon.
Considering Cale’s vitality is abnormal even by TCF standards (he can overuse ancient powers and still live through it, when it should kill anyone else), he wouldn’t just be a high-tier marechi—he’d be legendary, like the one-in-a-million source.
If quantified:
A normal marechi ≈ 50–100 humans
Cale ≈ 500–1000 humans minimum
With his vitality maxed out (say when he’s been “healing” or reinforcing his body with ancient powers), he could potentially equal several thousand humans.
Narrative Implications:
Demons would literally go insane in his presence; even Muzan might risk exposing himself just to obtain him.
Slayers might be terrified to let him fight on the frontlines, because if captured, it would basically doom humanity.
Ironically, Cale would hate all the attention because all he wants is a slacker life, but his very existence would destabilize the balance between demons and humans.
So yeah, that's the idea. Anyone who wants to add into this, please feel free.