Rewind Drug

The Rewind Drug is what Kyudai Garaki got when he looked at the Quirk-Destroying Drug closely enough to see the Quirk sitting underneath it. Working back toward what Rewind originally does, rather than the weaponized effect the Shie Hassaikai had built, he produced a single dose, usable exactly once.

He left it in a concealed cache of equipment intended for All For One and nobody else, packed alongside a voice recording that explained what the thing was capable of.

Take it and the damage reverses, all of it, including the long-accumulated harm that was supposed to be permanent. The catch is that there is no brake on the mechanism. The body keeps winding backward past health, past youth, until the person is simply gone.

Exertion accelerates the decline. All For One discovered this against Toshinori Yagi, dropping from young adult to child far faster than the drug otherwise moves, because he would not stop fighting and would not stop drawing on his Quirks.

It also differs from its parent drug in what it grips. The Quirk-Destroying Drug went after the Quirk Factor and nothing else. The Rewind Drug leaves the Quirk Factor alone and works on the flesh only, which is why All For One held on to his entire stockpile of Quirks the whole way down.

All For One is the only person to take it, and he did not do so by choice. Endeavor had already put him in a dead state; the drug hauled him out of it and returned him to his prime strength, though only for a time.

The bill came due against All Might. He shrank into a child, then an infant, then a fetus, then a single egg cell, and finally there was nothing left of him at all.