WARNING: The following contains spoilers for the My Hero Academia manga.
Out of all the many Quirks shown throughout My Hero Academia, one stands out as one of the most powerful and feared: All For One. This Quirk allows its user to steal the Quirks of others, after which said user can then utilize the stolen Quirks. It's the antithesis to One For All, the Quirk of All Might and Deku, in every way.
We first learn about All For One when All Might is explaining the history of One For All to Deku in Chapter #59 of the manga and Episode 33 of the anime. All For One was originally owned by a man during the rise of Quirks a century ago, and he used it to steal the powers of others and garner a large following. This man was known as Shigaraki, and later became the villain All For One.
Eventually, he became an evil dictator and ruled over Japan. His younger Quirkless brother was granted an ability to stockpile power by Shigaraki using All For One. It was unknown that the brother had his own Quirk already though; one that had the power to transfer itself to others, and so, the two Quirks fused, becoming One For All. The brothers eventually fought, and while the younger of the two transferred his ability to the next generation hoping that it would gain enough power to defeat the other, Shigaraki hid in the shadows, stockpiling Quirks.
With all that said, let's look at some of the strange secrets that make All For One just so dangerous and mysterious.
All For One Activates By Touch
Whereas One For All needs to be ingested to be passed on, its counterpart is activated by touch alone. The first time the ability is shown is when the students are attacked during the Vanguard Action Squad Invasion. Ragdoll, a member of the Wild, Wild Pussycats, has her Quirk stolen from her. We see other heroes have their Quirks also stolen throughout the series, too. This makes it a terrifying force to go up against, especially since there has been no way to restore the Quirks once they've been stolen (at the time of writing).
All For One Can Distribute Stolen Quirks
Not only can All For One take Quirks, but it can also then give them out to whoever the user chooses. Shigaraki used his power to distribute stolen abilities to the Nomu that Dr. Garaki created. Sometimes, this results in Quirks fusing, as mentioned earlier in the case of Shigaraki's younger brother. This can lead to the powers mutating and becoming even more powerful or creating new abilities. Dr. Garaki explained that without All For One, distributing stolen abilities requires surgery and three months of integration and recovery. This means that creating Nomu went more smoothly and quickly when utilizing this aspect of All For One.
All For One Can Be Duplicated
The version of All For One that Shigaraki uses is only a copy. He had the original duplicated, and took the copied version prior to the Kamino Incident. He did this with the intention of passing on the Quirk to his protégée Tomura Shigaraki -- real name Tenko Shimura. Duplicated versions of the Quirk have been shown to not be nearly as destructive as the original, but that doesn't make them any less of a threat.
All For One Has Three Users
All For One has had three people use its abilities, with the first being the original owner, Shigaraki. The second person was Nine, who received a copy of the original. His version wasn't nearly as powerful though. He could only steal up to eight Quirks, whereas Shigaraki has no such limitations. It is also unknown whether this version was also able to distribute Quirks. Nine had donated his body to the League of Villains for experimentation, which is where he received his diluted version of All For One. The third person is, of course, Tomura Shigaraki. He received the original from Shigaraki, thus receiving the full benefits of the Quirk.
All For One Does Have Weaknesses
Despite how powerful and terrifying All For One's abilities are, it does have weaknesses. The first is that once a stolen Quirk has been given to someone else, the user of All For One can't use it anymore unless they steal it back. The second is that the person the stolen Quirks are passed on to may not even be able to handle the transfer, resulting in them becoming a mindless shell of a person. It seems like there are ways to determine if a person can handle the transfer, like determining if the recipient already has a Quirk, so this drawback isn't as major as some of the others. This Quirk is also the embodiment of "jack of all trades, master of none." Some of the Quirks that are stolen may not be as useful because they require training and years of mastering. Finally, the number of Quirks the user steals can cause trauma to their body, eventually causing cellular degeneration.
Now that All For One has a new owner, we can only wait to see how the students of Class 1-A deal with this new threat. Not only did Tomura Shigaraki receive a Quirk that can steal others, including One For All, but his own Decay Quirk also became more powerful. We can only hope that eventually, he and All For One can be stopped permanently.
