Warning: contains spoilers for My Hero Academia chapter #355!
The ultimate villain in My Hero Academia was just defeated by a hero he already killed. All For One saw his own powers revolt against him as a consequence of absorbing Star and Stripe's quirk, New Order, which means that America's no.1 hero managed to defeat the villain from beyond the grave.
The most powerful superpower (called «quirks» in the series) shown in My Hero Academia is without a doubt All For One, possessed by the eponymous villain and by his «vessel» Tomura Shigaraki, which allows them to steal other people's quirks with a simple touch. Over the years, All For One stockpiled a great number of powers without issues, until he tried to absorb Star and Stripe (Cathleen Bate)'s New Order, another incredible quirk. New Order allows its user to «set a rule» onto their surroundings, letting them bestow new properties onto themselves and the world around them. Shortly before dying, Cathleen used this ability on her own quirk, basically turning it into a virus for All For One.
Related: My Hero Academia: Shigaraki Battles Star and Stripe in Epic New Fan Art
Star and Stripe arrived in Japan after the country fell into chaos due to the actions of All For One and Shigaraki. Star and Shigaraki immediately fought each other, and the American hero sadly fell in battle. However, with her last breath, she set a final rule: that New Order would revolt against other quirks. Once absorbed by Shigaraki, New Order starts destroying the other quirks contained in All For One, causing great damage before Tomura gets rid of it. It seems that New Order has finished its job, taking many quirks away from All For One and damaging Tomura enough to give the other heroes time to
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