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Sanegiyu/Slayer Mark Headcanon
The 25 year expiration date makes for some good angsty fic, don't get me wrong. I've written it myself, and probably will again. But it is so so tragic 🥺 Don't they (and Tanjiro, too) deserve to live full, happy lives after all the shit they have been through?
The Mark is tied to Muzan, just like the Ubuyashiki curse is. Where the curse is a punishment on the family line that produced a demon, the mark is the universe creating balance with the presence of demons by giving humanity a way to defeat Muzan.
Thus! When Muzan was defeated and demons were eradicated both the Ubuyashiki Curse and the Slayer Mark were also removed. The universe was brought back into balance.
So NO 😠They did not only get four demon-free years of normality and then die anyways. They were not punished for ridding the world of Muzan Kibutsuji. Sanemi didn't have sit and watch Giyu die and wait nine months for his turn. They lived long, happy, full lives, damn it, because I said so and it makes me happy to believe it.