It's not a coincidence or arbitrary. Satori rules the underground because she can dig up traumas whenever she needs to in order to control vengeful spirits. It's not "the one attack" Mizuchi is weak to, it's being sealed in the exact same way she originally was, forcibly reliving her past that causes the damage. It just hadn't come up because Satori kept getting jobbed everytime they crossed paths.Damn, Mizuchi's plan of doing all that convoluted possessing stuff just to fight Reimu in a one-on-one fistfight (with audience) at the end was perfect! But who could've guessed that Satori was such a big brain as to arbitrarily remember that one attack Mizuchi was weak against!?
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