One For All

Two brothers stand at the root of One For All. The elder, calling himself All For One, could take Meta Abilities and hand them out at will. The younger, Yoichi, appeared to have nothing. In a cruel gesture of affection, All For One pushed a power stockpiling ability onto him, not knowing Yoichi already carried a dormant Quirk whose only trick was moving itself into someone else. The two fused, and a power that compounds through inheritance was born.

Yoichi died at his brother's hands, but his blood landed on Toshitsugu Kudo, the resistance leader who had rescued him, and the ability took root there instead. Bruce Lee, Kudo's second, worked out what had happened: whoever receives the power also receives everything stored before them. He named it One For All with a note of sarcasm, betting that enough generations of growth would eventually produce someone able to end his enemy.

That bet ran through Hikage Shinomori, then Daigoro Banjo, En Tayutai, and Nana Shimura, most of them dying with the villain's hand on them and shoving the power at whoever stood closest. Nana broke the pattern by choosing an heir on purpose, a Quirkless boy named Toshinori Yagi. He became the first holder to beat All For One, kept the power forty years, then passed it to Izuku Midoriya. Izuku ended the line by forcing it into Tomura Shigaraki, destroying the Quirk along with him.

Handing the power over requires the successor to swallow some of the holder's genetic material, a hair or a few drops of blood, and it only takes if the current owner means for it to. Stain once drank Izuku's blood and inherited nothing. Willing transfer is not the same as a consenting recipient, though: All Might noted the power can be pushed onto someone who never asked for it. Because the will at its core refuses to bend to All For One, theft is off the table, which made it one of the few abilities usable against a man who could strip almost any other.

What it actually does is hoard raw force and dump it into the body, pushing strength, speed, agility, and toughness far past human limits. All Might's full power swings shifted local weather. Izuku's shockwaves tore into the alchemical monster Dark Might became while standing well back, and split a city sized fortress in half with one kick. Holders can flood the whole frame evenly or pour everything into one limb, which multiplies the output and the strain on that limb in equal measure. Once the power reached singularity, the Quirk Factors of earlier owners surfaced from its core, giving Izuku access to Gearshift, Fa Jin, Danger Sense, Blackwhip, Smokescreen, and Float, all amplified by a century of accumulation. The dead also linger inside as Vestiges, able to argue, advise, and lend their will, eventually letting Izuku reach an equivalent of 120 percent.

The costs are steep. An unprepared body simply bursts, so every heir needs punishing physical conditioning first, and Izuku broke his own limbs repeatedly before learning to spread the load evenly. Anyone who already owns a Quirk pays in lifespan, a full cup overflowing: Hikage took it at twenty two and died of old age at forty. Only the Quirkless escape that toll, which is why Izuku was always going to be the last. Copying it is useless, since an Accumulation type ability lends no stored power to a duplicate, as Neito Monoma discovered. A former holder keeps only fading embers that dim with every use.

Yoichi Shigaraki originated the power but was too frail to do anything with it, though his defiance became its permanent core. Kudo took it by accident and died grinning at the villain who killed his friend. Bruce named it and worked out its rules before passing it to Hikage Shinomori, who judged himself hopeless in a fight and spent eighteen years alone building the reserve instead.

Daigoro Banjo, once the Pro Hero Lariat, kept brawling with All For One's forces after most heroes had fallen, and was the first to shout that it was his successor's turn as he handed off. En Tayutai used Smokescreen to cover Nana Shimura's escape with the power. Nana raised Toshinori Yagi as her deliberate successor before All For One killed her.

Toshinori held it longest, forty years, spared the lifespan penalty because he was born without a Quirk. He went undefeated, beat All For One at the cost of ruined organs and a three hour daily limit, then chose Izuku Midoriya, another Quirkless boy. Katsuki Bakugo carried it briefly. Tomura Shigaraki received it last, forced into him to wreck his mind from within, and the accumulated century of power proved more than his crumbling body could survive.