Forgotten Enemy (Bnha x KnY)

Quirks are everything nowadays. Since the glowing baby incident, humanity has been obsessed with quirks. Technology stagnates, knowledge lost and hidden threats forgotten.
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What if in the midst of all this chaos, one of these old threat is reborn. A threat far more dangerous than any villain and far more dangerous than any quirks, waiting for the right time to strike. Its name, Kibutsuji Muzan, the demon lord.
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Warnings:
-Contains cursing
-Death
-ooc characters
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Canon divergence:
-Midoriya has a twin younger sister
-Izuku is abused for being quirkless by his mother
-Demons were largely forgotten except by a few...

I do not own Bnha or Kny, I only own the original characters in this story...