Warning: contains spoilers for My Hero Academia's 356th chapter
All for One's arsenal of quirks is one of the key reasons why he's such a dangerous villain, but that power and his desire to hold on to it may be what finally brings down My Hero Academia's big bad.
All for One takes his villain name from his original quirk, which enabled him to transfer quirks from one person to another, as well as implanting quirks in those without and removing them entirely. He used this ability to build his own power in society, and upon meeting Dr. Garaki, became able to not just take quirks, but clone them as well. In this way, All for One could keep a good ability and return it to its original owner, as he did with Garaki's slow-aging quirk, radically increasing his ability to steal and stockpile power. He's used these powers to give to those working under him like Lady Nagant, to create Nomu, and for his own use. However, after the All for One quirk itself was transferred to Tomura Shigaraki, it would seem that All for One's abilities should begin weakening, in just the same way that All Might's did. All for One knew this too, though, and took steps to prevent it, by giving himself a cloned version of the All for One quirk.
Now, Chapter 356 reveals this fact in the context of his fight against Endeavor, Hawks, Tokoyami, and Jiro. The cloned All for One quirk just can't match the original, apparently, as All for One struggles to control the quirks within him. Each quirk is equipped with a tiny piece of the original owner's soul--including former pro heroes like Ragdoll, and they're able to resist enough to slow All for One down and create openings for those physically engaged in battle with him.
The reveal that All for One is using a cloned version of his own quirk makes perfect sense; he's cloned so many other quirks an untold number of times that he would have no problem doing the same to his own. After all, his original body isn't meant to be around that much longer, since a big chunk of his soul was sent into Tomura Shigaraki's body alongside his real quirk. The only thing he didn't foresee was that the cloned quirk wouldn't be able to control the other quirks as effectively and that they'd be able to use that to rebel, much like Star and Stripe's quirk did when it was stolen.
In his attempts to keep from losing power as his body swap progressed, All for One inadvertently created a weakness that the heroes can now exploit, even if they aren't fully aware of what it is. It's a bit poetic that some of My Hero Academia's now-quirkless heroes like Ragdoll are going to be essential to All for One's defeat, given how little regard he's shown for anyone without a quirk. Defeating All for One's original body is just one step on the way to My Hero Academia's ending, however, so the heroes will have to save their celebration a little longer.
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