The fight was over as soon as it began. The villain dashed forwards, so fast that Izuku couldn't do anything to defend against it. He suddenly appeared in front of Izuku, not even touching him but still managing to throw him into the air with the force of his movements alone. As he flailed through the air, Izuku was reminded of a case where All Might had fought somebody with a fluid form, and had used overwhelming physical force and air pressure to break his body apart so that he could save the hostage the villain had formed himself around. Izuku had only learned later that the hostage had been Bakugou, and that the case had happened on the very same day that Izuku and All Might had met, when he'd broken Izuku's heart, so it was a bitter memory to recall as he came back down to the ground, landing flat on his back and having the wind knocked out of him. He slowly dragged himself to his feet, heaving and groaning, while he collected his thoughts, given time by the fact that the villain hadn't pursued him further, choosing to simply watch him with a massive grin-scowl on his face.

What was he doing? This guy could easily kill Izuku, so why was he just choosing to … play with him? It made no sense. Izuku quickly ran through what he knew; this villain could supposedly hold multiple Quirks, he was called All for One, All Might's Quirk was far stronger than any single Quirk had any right to be, this villain had a grudge with All Might, having fought him before and barely survived, the villain was after something called 'One for All', which was thought to belong to All Might, but now the villain has reason to think that he passed it on, and that Izuku was the one he gave it to. All of that painted a pretty clear picture. All Might had a transferable Quirk, and this villain, All for One, wanted it so badly that he was willing to go through all of this just to get it. One for All sounded like a massive Quirk for anybody to have, even for someone who's already as powerful as the top hero. Izuku was pretending to have it so that he could buy All Might some time, but he really didn't have anything like that in his arsenal, and if he didn't show off some degree of super-strength soon, if that was truly what One for All was, then All for One would likely figure out what he was doing and stop letting him live just because he had a Quirk he wanted.

"Hah, this is your successor, All Might? A child who is not even using it, even in a situation as dire as this one. The first half-dozen defilers were not especially powerful, and it was only until your waste of a master that you people could even hope to stand against me, but I would have thought that any successor of yours would take any chance they got to use it. Even further, it should be hitting singularity any day now, so this boy should be showing signs of any number of Quirks, but there's nothing. Just that pesky Quirk that dusted my Nomu," the colder voice said. "What a disappointment. This brat isn't even using One for All, and he's on his last legs. Sure, it wasn't so strong until recently, but could it be too strong? I wonder if he can't use it. It would make sense, since it should'a popped right about the ninth or tenth user, given how much raw power you added to it, All Might," the higher voice cackled.

Izuku took his chance to act while the villain was distracted talking to All Might. He knew the villain wasn't actually distracted, he knew that he was just trying to belittle both of them and get into their heads, and was probably even more aware than usual while he was monologuing, but Izuku could do this. He had a plan. He began running at full speed, which wasn't very fast in his tired, injured state, and threw his fist back as if he was preparing a punch. The villain snapped his attention back to Izuku, that same half-grin, half-scowl on his face. It seemed like the villain began shaking at that moment, as the two differing expressions on his face only became more different, like each of them was expecting something and was dreading and looking forward to it at the same time. Izuku couldn't pin down an emotion to attribute the shaking to, so he decided that it must be one of the things that both … people? Personalities? He really didn't know what this guy's deal was, and he wasn't sure he wanted to. He kept running at the villain as the tall, white-haired man spoke again, not taking his terrified, gleeful eyes off of him for a second.

"Finally using your trump card? I should feel honoured, then? The great successor to the Symbol of Peace, what a joke!" the higher voice spoke, surprisingly the more dominant of two now. "Oh, what a shame, things were just getting interesting, I was just about to share something with you that I think you would find interesting, All Might!" the lower voice yelled, seemingly in a last ditch attempt to assert his control over his host body.

Izuku noticed that the villain wasn't dodging, or preparing any kind of counter to his attack. Maybe he was planning on tanking it? He probably could, but Izuku had to make it out like he had some kind of super-powerful ace up his sleeve, and All Might-level strength was certainly a trump card, especially after the villain had just been fighting that same strength. Izuku reached the villain and then threw both hands back, feeling his bandages evaporate on his right hand and his glove on the other. He then plunged both hands forward onto the villain, slapping them onto his bare chest and completing the circuit needed to activate his Quirk on both hands. Decimation spread out in a devastating pulse from the man's chest, and spread all the way to around the shoulders and the lower torso before it slowed to a crawl and began to be pushed back by what was seemingly another healing Quirk. Waves of Decimation raced over the villain, the skin and muscles turned to ash and dust and immediately healing, just to be Decimated again. Izuku pushed his hands further into the man when he could, lodging his hands inside the villain so that his Quirk worked from the inside out, causing the villain to be paralysed in an odd way, just like the Nomu.

That cycle continued on for a few moments, with Izuku screaming as he finally unleashed himself again and got some of that burning energy out of his body, putting his everything into trying to accelerate Decimation along and reduce the villain to nothing, even if he knew it didn't work that way. Izuku wasn't sure if he'd ever be able to control the speed at which Decimate works, even if it had gotten faster on its own before. It wasn't something he could train, but he could put himself in positions where it was more or less effective, like shoving his hands into the villain's torso between fluctuations of his regenerative Quirk. The villain was screaming, too, but he was actually saying words, as opposed to Izuku's incoherent, animalistic roar of effort and pain, as his injuries were aggravated by the attack.

"Uh-oh, All Might, look at this!" the cold voice said, the higher voice still screaming. "Your successor is killing my host. What a shame, because I'd planned to use him for quite a while longer. After all, Tenko Shimura is someone who is very special to both of us, wouldn't you say?!"

Izuku looked over to All Might, still keeping up his attack, to see a frozen, broken man. He suddenly looked twice his age, and the look in his eye was almost as harrowing as the ones he'd give in his true form. His mouth was hanging open, jaw dropped in a disturbing show of fear and denial. Izuku had never seen All Might look so lost, even that day on the rooftop. Who was Tenko Shimura? Obviously, it was someone important to All Might, because of the reaction, but it wasn't the name of any publicly disclosed family, and … Izuku felt stupid. Of course there wouldn't be any publicly disclosed family of the number one hero. He stopped his verbal assault on the villain and fell as silent as All Might at the disturbed, mistrusting look in the hero's eye, the only remaining sound in the plaza being the host, Tenko Shimura's screaming.

"Shimura? No, that's a lie. Where did you even …? My master had no family!" All Might said, getting to his knees from on the floor at last.

"Not publicly, but I have ways of getting information that others can't. That's how I found out about poor Tenko here, and how his family abused him. Looks like Little Kotaro didn't grow up to be the biggest fan of heroes after all!" All for One laughed.

"But why didn't … Kotaro. I've heard that name before, but I … she never … why?!" All Mights stumbled over his words, his voice catching on unfelt emotion.

"Well, it wouldn't do to have the most powerful villain in Japan go after her son, now, would it? So she abandoned him, left him to grow up without a true mother. Kotaro felt let down by heroes, because his own mother had given him up to be one. When he grew up, Kotaro had a son, Tenko. He was born into a home that hated hero society, but poor little Tenko wanted to be one himself. His father beat him, All Might, berated and abused the boy until he gave up on his dreams. At first, I was just going to steal Tenko's Air Walk Quirk and use it when we fought next, defeat you using a power that was so very similar to your own master's, but then I saw the hate he carried in him, the potential lurking in his heart, and so I gave him my own Quirk. I saw how my little brother has come to fancy successors, so I made Tenko Shimura my own successor, the next me. Now, he is what you see before me, completely consumed by his hate for you, and currently being ripped to shreds by your pupil! How about that for peace, All Might?!" All for One continued over Shimura's screams.

All Might was completely still, kneeling on the ground as he stared, open-mouthed at the display before him. Izuku's hands were beginning to feel numb, and he knew he'd have to stop sometime soon. All Might just needed to get up and keep fighting, that was all! Izuku knew that he was going through a tough time right now, but he needed to pull himself together! Izuku was counting on him, the entire class was counting on him, and if All Might couldn't stand up and fight right now, then what was he there for? Izuku took a deep, steadying breath, and screamed.

"All Might! Please, get up! You can't just shut down! I need you to help me do this!" he yelled, but it was mostly lost under Shimura's constant, never-ending cries of pain and something else that made Izuku's skin crawl.

"What about that, All Might? Do you still have the will to fight when you know you'll be putting down your master's own flesh and blood? Huh?! Answer me, you buffoon! Are you really going to let your useless successor die?!" All for One screeched.

The villain then began moving with what sounded like great effort. The bones in his arm all but creaked with how much power he was putting into his slight movements, and it felt like the air was heavy, just like the other times he'd displayed his awesome strength. The villain was going in for an attack, and Izuku got the feeling, the gut feeling that only someone in mortal danger got, that Izuku wouldn't be getting back up if he took this one head-on. It all then played out in slow-motion in that split second where everything seemed to happen at once.

The villain hadn't been going for an attack, simply placing his hand over Izuku's face gently. Then, an excruciating pain surged through his body like nothing he'd ever felt, a pain worse than Thirteen's Black Hole ripping through his flesh, and it was all from the inside out, not the outside in. It was as if every cell in his body was trying to free itself and go in a different direction than every other one, and it was a pain so intense that Izuku was blinded by it for a brief moment. Then, all he saw was red. It wasn't because he was angry, though he was still consumed by this unending urge to beat this villain. No, this was different. It was as if there were red arcs of lightning pouring out of Izuku, bolts of raw power and weight that felt like all the tension in his body was leaving him. That red lightning surged into the air and was held there for a moment as Izuku's body went slack, his hands stuck in the villain's torso and still Decimating him, but to a lesser degree and quickly decreasing further. It was like his very essence was being ripped out of him, and Izuku almost passed out again, but then the second thing happened.

A massive blast of air whipped Izuku's hair out of his face and kept him awake. The villain was sent flying away immediately, the pressure around Izuku's hands releasing from all angles, and all the vitality and strength that had been taken from him was suddenly all given back, as if he'd been healed like in a video game. He looked and saw All Might standing there, tall and strong, but still bleeding from the side of his chest. He had his fist raised, probably meaning that he'd attacked the villain once more, and Izuku felt himself smile before looking down at his hands to make sure they were still up to the task of using his Quirk. They were, but there was another thing that caught his attention.

The legs of the villain were there, but his torso wasn't. The split likely happened at about the point Izuku's hands were touching him, around the middle of the torso, which meant that his lower half dropped to the ground in a heap of impossibly powerful muscle and bones. Some of his guts fell out and onto the floor, and Izuku looked away to keep from throwing up. He looked up to see the villain's top half flying through the air, the two voices cackling in sync once again. He was acting as if he hadn't just been punched in two, and honestly, that was what made Izuku throw up. How inhuman was this All for One if he could laugh off having no legs? The villain seemingly tried to stop in mid-air, but quickly fell and hit the ground with a loud, wet impact. He called something out, and Kurogiri suddenly appeared, making Izuku wonder where he had gone the whole fight.

"Yes, Master?" Kurogiri asked, as polite as ever, even though it looked like he had a little bit of blood on his metal neck-brace.

"A change of plans, Kurogiri. We are retreating, as All Might and his successor have injured me beyond immediate repair. Warp us out, now!" the villain said, their two voices still in sync.

Izuku looked closer, and it looked like his healing Quirk was doing something, with the skin growing and the bones and muscle coming in very slowly, but still enough that it could be considered active. It looked like that amount of damage was taking its toll on both the villain's body and his regenerative Quirk after the damage he'd already taken in such a short timespan. Izuku silently felt his heart leap in his chest as he heard that they were going to retreat, but then he processed the blood on Kurogiri's brace, and felt a pit form in his stomach at the thought of who it could've belonged to. It was entirely possible that it was Kurogiri's own blood, but a dozen different possibilities popped into Izuku's head at that time, and he didn't have the peace and quiet to sort through all of them without panicking.

And then they were gone, as simple as that, with Kurogiri and All for One warping out of the USJ and the central plaza falling silent. All Might immediately deflated to his true form, and then swayed slightly on his feet, as if he was going to fall over. Izuku suddenly felt as though his limbs were heavier than the earth itself, and felt a great pull tugging him down. He looked over at All Might and saw the tall, rake-thin man looking down at him. There was a silent question in his eyes, the implication of concern and curiosity, and Izuku would love to dig into what had just happened, but he was fairly sure that his eyes were closing and he wasn't doing it on purpose.

"We're gonna talk," Izuku said, his voice hoarse from screaming.

Then, he fell to the floor with a dull thud, the sound muffled in his ears as he finally felt the adrenaline fade from his system and the crash settled in. He'd done it. He'd saved All Might, and he hadn't died to do that. That'd show him. That'd show them all that he could be the greatest hero that ever lived, and he could do it with his evil power. It was just too bad that All for One and Kurogiri had gotten away. They'd be problems later, but for now, Izuku could appreciate the feeling of having saved the number one hero from certain death, and proved to him that he was wrong. Although, Izuku had a feeling that he'd be hearing from Aizawa about all that he did, and that he was in for a shouting match with his mother when he went home, though he had no clue when that would be. Overall, it was nice just to know that he'd done it.

Izuku was a hero that day, and nobody could say otherwise.