The introduction of Quirks to the world led to a period of societal collapse and an age of upheaval. During this time, a man with an overwhelmingly powerful Quirk called All For One arose in Japan and decided to take advantage of the chaos. He stole many Quirks, attaining immense power and influence. Using his incredible might, the man gathered and unified dozens of people under his command. He then manipulated the people he gathered like pawns to further his plans and eventually became Japan's ruler. It's suggested that he ruled from the shadows, as most of his actions in the present day are only known as rumors on the internet.
All For One's younger brother opposed his evil as All For One tried to show him that he was helping people by ridding those who believed themselves plagued and giving strength to the helpless.
All For One's brother saw through All For One's claims, deeming that he is only trying to amass people who owe him into servitude. All For One defended this by stating his Quirk is able to return order to the world that has lost its humanity at the hands of Quirks. All For One's brother refuses to give in to his brother's manipulative statements and tried to rebel but was stopped by All For One's personal bodyguard. All For One berated his brother for not having a Quirk, but says that he still loves him as he is his only family and imprisoned him.
When All For One visited his captive brother, he saw how thin he become due to not eating. All For One tells his brother about how a group who refused to join him were killed, simply because his followers interpreted his desires as instructions to kill them. All For One's brother was disgusted and voiced his defiance. Enraged, All For One used his Quirk to forcibly give his little brother a Quirk that could stockpile power. What no one knew at the time is that All For One's younger brother already had a innate Quirk which could only be transferred to others. The two Quirks mutated, blending together, and would later become known as One For All.
All For One's younger brother was not strong enough to defeat him even with his new Quirk, so he passed it on to the next generation in the hopes that one day One For All would be strong enough to defeat his older brother. It is suggested that All For One would fight these successors.
Seven generations later, Sensei killed All Might's predecessor, Nana Shimura. All Might eventually fought and critically injured All For One, but he similarly received a debilitating wound in the process.
Incapacitated by his defeat, All For One went into hiding as he attempted to heal from his injuries. He would later create the League of Villains (where he took on his "Sensei" identity) and begin grooming Tomura Shigaraki as his successor. He was well aware that Tomura was really the grandson of Nana Shimura and wanted him to use this to his advantage. He also started grooming other powerful individuals who he deems to have a lot of potential, such as Gigantomachia.
Kurogiri warps himself and Tomura back to their hideout at the last second, allowing them to escape from being turned to dust. Tomura complains that his body hurts and all their subordinates were eliminated, Nomu was defeated and All Might is still alive, further complaining that everything went wrong. However, All For One, speaking on the monitor says that nothing went wrong and that he was simply over optimistic. All For One says that it is too late to mourn and that they should gather a new group hand picked for efficiency, saying that they should take all the time they need. All For One says that Tomura is a symbol of importance and that next time he will show the world the horror of his existence.
A recuperating Tomura was seen watching the Obstacle Race with his computer and he sees that Izuku took first place. Later on, before the battle between Izuku and Shoto begins, All For One tells Tomura to pay close attention to Izuku and Shoto's match as they might become obstacles for him in the future. Tomura responds by telling the Sensei to not make him laugh.
All For One, using the monitor to speak through, orders Kurogiri to allow Stain and Tomura to continue talking, saying that Tomura's meeting with Stain (an experienced infamous villain) will stimulate Tomura's growth. Before he leaves for Hosu, Tomura asks All For One to let him borrow some Nomu because he does not like the "Hero Killer" Stain and that destroying things he does not like seems alright. All For One decides to give Tomura three Nomu, hoping that Tomura will make the opportunity a learning experience.
All For One is surprised that the Hero Killer Stain was captured but is still happy that things are going as he has foreseen. He exclaims that Tomura's position is to supervise and unify the new villains who have come to join the League of Villains. All For One's doctor expresses doubt that Tomura can do his job and says that if All For One stepped in things would be proceeding faster. All For One asks the doctor to patch him up faster. All For One says that he will let Tomura do all the hard work so that he may become his successor as Tomura was born to do so. All For One tells All Might to rejoice in the "transient" peace while he still can.
After Katsuki attacks Tomura, Tomura understands that mere words alone won't convince Katsuki to join them and decides to ask for the "force" of his teacher, All For One, to help him convince Katsuki. All For One, who has been watching Tomura on his computer, commends his apprentice for his wise decision.
The League of Villains is apprehended by the Pro Heroes and Police Force. All For One decides to take action himself and appears at the League of Villains' warehouse and apologizes to Tora, explaining that Ragdoll is in a comatose state because he stole her Quirk, Search, which he found to be quite useful. Best Jeanist restrains All For One with his Quirk and orders his fellow pro heroes to not let the villain do anything.
Before the pro heroes can act, the entire warehouse is completely destroyed and the surrounding area suffers extensive damage as well. It is revealed that All For One unleashed a blast wave that has seemingly killed the pro heroes, destroyed the warehouse, and dealt significant damage to the surrounding area in less than a second. All For One is livid with the pro heroes for interfering with his apprentice, Tomura who has started thinking for himself as well as leading others by himself and hopes they no longer bother him. All For One decides to quickly resolve the mess created by the pro heroes.
All For One uses a Quirk to produce black liquid at the League of Villains' hideout; the black liquid summons dozens of Nomu inside and outside the hideout, forcing the pro heroes and Police Force to confront the legions of Nomu. This black liquid also teleports Katsuki away.
All For One sees that the pro heroes survived his attack. All For One claps and praises Hakamata for using his Quirk to pull his fellow pro heroes out of the way in order to reduce the blast's impact on them. Despite his heavy injuries, Hakamata attempts to fight back and begins sending fiber strings. However, All For One impales Hakamata from afar. The mighty villain mentions that he does not need Hakamata's Quirk because it does not suit Tomura’s personality.Katsuki's materialization from the black liquid in front of All For One catches the attention of Izuku's group. All For One apologizes to Katsuki as Tomura, Kurogiri, and the Vanguard Action Squad materialize from the black liquid as well. All For One sees that his apprentice, Tomura, has failed again. However, All For One is neither angry nor disappointed with Tomura and encourages Tomura to try again since he still has the Vanguard Action Squad and Katsuki whom he deemed important. All For One implores Tomura to continue trying as many times as he likes since everything is for his sake.
Suddenly, All Might appears above him and attacks him. However, All For One blocks All Might's attack. All Might declares that he will take everything back while All For One wonders if All Might will attempt to kill him again.All Might and All For One clash, which creates a shockwave that sends Katsuki, Tomura, and the Vanguard Action Squad backwards. Both All For One and All Might criticize each other for becoming weaker. All Might declares that he will save Katsuki and will not repeat the same mistake as he did five years ago and this time he will definitely put All For One behind bars along with the League of Villains. All Might charges at his arch nemesis. However, All For One sends a huge blast wave at All Might, causing the Symbol of Peace to be sent flying away with great force. All For One's blast wave attack not only hits All Might but destroys several buildings in the process.
Katsuki becomes concerned, but All For One tells Katsuki not to fret since his blast wave attack is not strong enough to kill All Might. All For One orders Tomura to escape along with Katsuki and comments that his black liquid warping Quirk will not help Tomura and the League of Villains escape. All For One uses a Quirk that forcefully activates Kurogiri's Warp Gate, which causes a portal to open. All For One orders the League of Villains to escape into Kurogiri's portal. Suddenly, All Might reappears onto the battlefield, refusing to allow the arch villain to escape and charges at him. Before heading into battle, All For One tells Tomura that he can still grow stronger.
All Might and All For One clash once again. Meanwhile, Katsuki is fending off some of the Vanguard Action Squad; All Might attempts to help Katsuki only for All For One to intervene and stop him. Suddenly, Izuku, Eijiro, and Tenya appear above in the air; All For One sees the trio and attempts to attack them, but All Might intervenes by punching him.
Katsuki manages to escape along with Izuku and his friends. Now that the tables have turned, All For One sees that they no longer can secure victory due to Katsuki's rescue and that most of the Vanguard Action Squad is unconscious. All For One decides to save Tomura and the Vanguard Action Squad. All For One forcefully activates Magne's Magnetism. Before Tomura can attack Gran Torino, he and the Vanguard Action Squad are magnetically pulled into Kurogiri's Warp Gate. As he is being sucked into the Warp Gate, Tomura calls out to his teacher concerned for his safety. Before Tomura teleports away, All For One tells Tomura to continue the fight. Tomura and the unconscious Kurogiri are teleported away by the Warp Gate, causing the portal to disperse.
All Might charges at All For One, but All For One uses his Warping Quirk to teleport Gran Torino in front of him, using the elderly Hero as a human shield which causes Gran Torino to take All Might's attack instead. All For One expresses his hatred for All Might as he is the one who brought down his comrades in the past and while he descended into darkness, All Might rose to the top as the Symbol of Peace. All For One prepares to fire his blast wave move, intending to blow away Gran Torino and All Might at the same time. However, All Might pulls Gran Torino away and uses a Detroit Smash to cancel out All For One's blast wave move.
All For One sees that his destruction of the city has brought chaos and decides to put the innocent civilians in danger so that All Might will not be able to fight at full strength. Furious with All For One's sadism, All Might grabs his arm while throwing Gran Torino away for safety. All Might admonishes All For One for all the despicable crimes he has committed and for his misanthropy.
All Might punches All For One in the face, shattering his mask; however, All Might's punch was not strong enough to finish off All For One. As Gran Torino calls out to him, All Might's face reverts to its true form. All For One says that All Might would not be the first to criticize him as he has heard those same lines before, from the previous One For All successor: Nana Shimura.
All Might is angry at the arch villain for mentioning her name. All For One notes that Nana was all bark and no bite, only going on about her ideals, and was not even strong; he mocks her for her uselessness and mentions that she died in an embarrassing, shameful way. Furious, All Might prepares to attack All For One. However, All For One uses his blast wave technique against All Might, causing the hero to be blasted into the air. Gran Torino catches All Might and lands him to safety on the ground.
All For One decides to finish his apprentice's hard work and sever the trust in heroes once and for all. He once again expresses his hatred for All Might due to taking something away from him and wants him to die in the most gruesome way possible. He fires another blast wave attack at All Might, who, seeing that an innocent civilian is trapped under rubble behind him, blocks All For One's blast wave attack and saves the civilian from being harmed. However, All Might expends the last of his power and reverts to his true form. All For One relishes in the fact that All Might's true form has been exposed for all to see.
Everyone around the country sees All Might's true form and begins panicking. All For One mocks All Might for the pitiful state he is in. However, All Might does not care about his rotting body nor his exposure to the world and declares that his spirit burns strongly which is what truly matters. Wanting to break his spirit, All For One reveals that Tomura is Nana's grandson. All Might is demoralized and refuses to believe, but All For One tells him that it is the truth. All For One wonders why All Might is not smiling and sarcastically remarks about All Might's smile, which is something Nana would always do. All Might's spirit is greatly demoralized by the mockery while All For One continues relishing in All Might's pitiful state.
Invigorated by the civilian, All Might activates One For All throughout the right side of his body, transforming it into its hero form. All Might smiles and states that Heroes have many things to protect which is why they do not lose.
All For One deduces that All Might will unleash his final attack, commenting that wounded Heroes are the most terrifying of all. All For One's comment causes him to reminisce about their last battle where All Might delivered the final blow to him and admits that the look on All Might's face during that last attack has still haunted him to this very day. All For One prepares to fire his blast wave attack. Suddenly, flames strike at the mighty villain, but All For One uses his blast wave attack to swat away the flames with ease. All For One is surprised that the dozens of Nomu he sent were dispatched with haste even though they were mediocre and should not have taken Enji's No. 2 rank so lightly.
Annoyed with Enji and the other pro heroes' interference, All For One prepares his blast wave attack again. However, Edgeshot uses his Quirk and great speed to attack, but All For One dodges the attack. Growing tired with the sentimental talk, All For One blasts Enji and Edgeshot back and activates a number of his strongest Quirks in his right arm, causing it to become an enlarged mutated arm filled with numerous deadly screws and hardened minerals.
All For One charges at All Might with his heavily empowered and mutated arm. While heading towards All Might, All For One knows from battling him that All Might no longer has One For All. He tells All Might that he knows to whom he gave One For All: Izuku Midoriya, having deduced it from Tomura's report about the boy. All For One taunts All Might by saying that he has also failed as a teacher towards Izuku due to his apprentice's recklessness.
Both All For One and All Might clash by punching each other with their enhanced fists, which creates a huge shockwave that devastates a great portion of the city. As they struggle, All For One uses his Impact Recoil Quirk to revert all the damage to All Might. However, All Might transfers One For All to his other arm, sacrificing his right arm but renders Impact Recoil ineffective. All Might says that he must indeed criticize Izuku for his actions. All For One is shocked that All Might would go to such lengths to protect his leftovers of One For All and admits that he miscalculated. All Might refuses to die until he has finished bringing up Izuku. He then counters by throwing his empowered left arm and punches All For One who has left his guard open.
The punch, however, was meant to lower All For One's guard, and All Might transfers One For All to his right arm and prepares for his final attack, United States of Smash, which creates a great shockwave when it hits All For One. After the dust clears, All For One is in a crater, badly beaten and unconscious. Later, Enji and All Might watch All For One being placed in the iron maiden.
The next day, All For One is incarcerated into Tartarus, a special prison for criminals for whom the death penalty is not enough. All For One, in a wheelchair and on life support, wonders where he is, much to the prison guard's annoyance who is pushing him to his holding cell; the prison guard tells him his location. All For One apologizes, causing the prison guard to assume that he is unable to see. All For One reveals that for the past six years he has been using an Infrared Quirk to sense his surroundings as a way to compensate his blindness, but due to all the sensors he is unable to sense anything and apologizes to the prison guard. The prison guard is flabbergasted that All For One fought All Might in such a condition.
While being pushed to his holding cell, All For One is not angry or upset for losing against All Might. Instead, All For One is happy because his defeat at All Might's hands will grow Tomura's hatred for All Might as well as his regret for being unable to save his sensei, which will motivate him to gain experience and become much stronger. All For One smiles, knowing that even though he lost the battle, he still will succeed as being the better teacher than All Might. All For One says to himself implying that Tomura is the successor who will inherit the All For One Quirk.
During the Hero License Exam, All for One receives a visit from All Might, who wishes to settle matters between them.
All For One claims he is willing to restore Ragdoll's Quirk back to her but that it would require activating his own Quirk to do so. The authorities determined the safest approach is to not let him do anything while investigating the number of Quirks he has stockpiled.
A few days later, Izuku Midoriya experiences the vestiges of One For All where he views a flashback between All For One and his brother, the first user of One For All. He witnesses All For One transfer the stockpiling Quirk on his brother, out of kindness as well as to force him to submit. All For One claims that he was creating a reality just like the one in the comics the brothers used to read as kids. He also tells his brother that by acquiring a Quirk he would start seeing the reality as it is. It is later revealed that All For One's brother was actually communicating with Midoriya.