All there was is now dust. That is the world Deku now lives in, dust and ash, all because of a simple mistake, a misinterpretation of Shigaraki's character, allowing him to destroy the world.
Izuku stands over a defeated Shigaraki; it was their last battle, the end of All for One, and the beginning of the end of modern villainy and the rise of the newest symbol of peace. Or at least it was meant to be.
"Ha...ha...ha, you haven't won, you know that," came the words of a beaten Shigaraki.
"I've won, you're defeated. All for One is dead, and your army is gone. There is nothing left for you but prison!" As Izuku said these words, a sickening laugh came from the maw of Shigaraki.
"Hahahahahah, you think this is the end! I haven't even used my special move yet! Do you think decay was the only quirk sensei gave me?!"
"What are you on about, Shigaraki?" With a grin, Shigaraki explains, "Amplify, a quirk that can be used once a day to double the power of another quirk, but a small detail that is overlooked: the boost increases exponentially the longer it isn't used, and I haven't used it since I received it ten years ago!"
In shock, Izuku says, "But that could decay the whole world!!!"
"Do you think I care? I destroy things I don't like, and... I think a rage quit would be an appropriate end to this shitty game," as Shigaraki says this, he slams his hand to the ground, activating his quirk, causing a wave of grey to spread from his hand, engulfing the horizon and far beyond.
Meanwhile, Izuku dives at Shigaraki, trying to stop him, but it's just too late.
"Nononono...NO!!!"
"Hahaha, I win!" Was the last word of Shigaraki as his body falls into grey dust, with a rictus grin plastered on his face.
A/N: this is mostly a chapter setting up the setting and for how izuku survived the attack will be explained in another chapter