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Decay

Summary:

All For One found Tenko Shimura and from the ashes of his existence, he created Tomura Shigaraki, his successor. Only I'm not Tomura Shigaraki. I'm not even from this world. I just woke up in the head of the leader of the League of Villains, with nothing but my knowledge of My Hero Academia to keep me safe. Somehow I've got to pass as Tomura, be a good villain, and prevent All For One from killing me if he finds out the truth... Tomura Shigaraki Self-Insert, 1st Person POV

Notes:

(See the end of the work for notes.)

Chapter 1: I Draw The Short Straw

Chapter Text

When I opened my eyes, I could tell with great certainty that something had gone horribly, horribly wrong.

I shook my head, wincing slightly as I felt a pain in my skull, and blinked my eyes a few times to try to clear my vision of the fuzziness that had descended on me. The roof of my mouth felt dry, as if someone had lit a bonfire on my tongue and let it die out, and my lips felt more parched than they had ever been in my life. Add into this the splitting pain that rang around my sinuses, akin to a migraine, and I was not feeling great in that moment.

That wasn't why I could tell that something was horribly wrong, however. The pain was a minor inconvenience compared to the crushing realisation, as my vision cleared and I became aware of the sensation underneath me, that I wasn't where I last remembered I had been. I remembered being outside in the fresh air, the smell of grass and mud after rain, hearing the sound of the ducks on the pond near the library where I had been studying. And then...

Then I didn't remember. That was wrong.

Maybe that wasn't a fair statement. I could remember... something. Fragments, as if someone had projected my life onto a pane of glass and then smashed the glass, so that all over the floor tiny pieces of broken glass were showing me parts of my life. I tried to focus, and I just... couldn't get the key details.

I thought of my parents, and I could only see the blurry silhouettes of two people in my head, associated with the feelings of love and admiration and frustration that go hand in hand with parents and children. But then I felt something else, saw something clearer, saw a face I didn't recognise and didn't associate with the idea of "parent". The severe face of a stern man, with sharp features and short hair, stared back at me, and I was none the wiser as to why, or why I felt such strong feelings of anger and resentment boil up in me. This man wasn't my father. Why did he appear in my head when I tried to picture my dad?

Whatever had happened, I felt disjointed, broken, confused. Something wasn't right and I didn't feel right in the head. This didn't feel like a hangover or anything like that; this was something fundamentally wrong. And that wasn't the only thing that was wrong.

If I had been asleep on my home on my bed, I... well, I couldn't even get a clear picture in my head of my own bedroom right now. I could see details in fleeting; I could picture a huge window looking out onto a sprawling back garden, a double bed with a spring out of place that dug into my left kidney if I lay on my back, and a movie poster somewhere on a cream-painted wall. I couldn't even remember the movie even if I really focused. But this wasn't it.

This room was dark, one faded orange bulb overhead which flickered as if the power wasn't quite there. The walls were dark brown and orange, and smeared with something I couldn't identify, and didn't want to. This was a single bed, surprisingly comfier than what I was used to, but far too short, so my feet poked over the end and hung into space. Between that and the rather strong smell of stale alcohol, this definitely wasn't my place.

"This isn't my room."

I spoke aloud, to myself, and there came the next revelation. "And that's not my voice."

I sat upright and tried to process what I'd just heard, how jarringly familiar it all was. My normal voice was quite deep, and I remembered that friends and colleagues had told me it always seemed to have an air of sarcasm to it even when I was genuinely trying to be nice to them. The voice I had just spoken in had spoken in a language which I understood but which wasn't my own. The voice was sharp, raspy, almost nasal in places, and that wasn't just the result of a dry mouth or dry lips.

There were all sorts of complicated emotions in that tone; I could hear real venom in it, the voice of someone holding a grudge, but there was also a sense of... whininess? How else could I describe it? Entitlement and hurt, as if I was about to throw my toys out of the pram because I'd spoken with a voice that wasn't mine. As if I deserved better.

Why did I know the voice?

"Okay, what bullshit is this?" I asked myself, not expecting an answer from anyone, and I decided to take stock by looking at what else I had to offer. At that point I looked down my body as I lay there on the mattress, and got confused even more.

These clothes weren't mine for a start; I hated skinny jeans, but apparently I had gone for a lie down in black pants which didn't even reach my skinny ankles, and hadn't bothered to take the red trainers off I was wearing. Those I approved of, and the black hoodie I was apparently wearing was incredibly comfortable, if not my style. This wasn't the pressing issue though; that, in fact, was my body. My legs and arms were too skinny and I was too thin, but the worst was my hands. I looked at my hands for only a second, with the nails nearly ink black and the skin a deathly pale yellow, and nearly screamed.

These weren't my hands.

"What the hell happened to me?" I said to nobody in particular, before noticing on a table besides the bed there was a glass of water. Thank god for small mercies- I could at least stop my tongue from feeling like sandpaper. I reached out...

And as I grasped the glass of water in my hand, it crumbled to dust in a moment.

"What."

I looked at the dust on the side of the table, floating in a little pool of water that had until a moment ago been my drink. I looked at my hand, outstretched where all five fingers had touched the glass. I looked at the dust again, and then I made the worst mistake of all. I looked at the other object on the table.

For a moment I thought that I was being kept company in that bedroom by a giant spider, until I counted the number of limbs. It couldn't be a spider with only five. Then the part of my brain that was still struggling to work out how I had disintegrated the glass caught up with the rest of my brain, and saw what the grey and blue abomination was, with its little box at the base and creepy outstretched fingers. There was no mistaking it- that was the mummified hand of a dead person, sat on my table like an ornament, its fingers pointing towards me. Beckoning.

It was time to take stock. I was in a room that wasn't my own. I had skin that looked dead and arms that looked like they were about to snap like twigs at any moment. I had hands which turned whatever they touched to dust, the whiny voice of a petulant brat angry at the world, and a zombie hand sat on my bedside table like it was my most prized possession.

And then it hit me. The sudden realisation. The memories that came flooding back of hours glued to a screen watching heroes and villains duking it out with everything at stake. Of the times curled up in bed reading the latest chapter, wondering what the hell was going to happen next. Heroes that made you question what it meant to be a hero, villains that made you question if they were wrong, stories of heirs and successors and the intertwined fates of good and evil. Stories to last a lifetime, like no other.

Stories of a young hero to be, and a villain like no other whose touch turned things to ash.

"You can't be serious." I jerked upright with a thud, landing on my feet far lighter than I remember myself being, and stared around the room, desperately hoping my theory wouldn't be right. With a horrible feeling in my gut, I saw the sink in the corner of the room, the cracked mirror on the wall, and I strode over. I didn't want my theory to be confirmed, but with everything else I had I was almost resigned to it, a nightmare becoming true.

I stared into the mirror, and Tomura Shigaraki stared back at me.

"Right. I know you love your games, Tomura, but this glitch is weird even by your standards."

I cracked a smile at my own joke, and chapped lips broke into a crooked smirk with faded yellow teeth. That wasn't going to be a face I could get used to anytime soon.

Right. Think. What did I know? I had devoured the manga back in my old life, and had tried to convince anyone I know who may be interested that they should watch the show. Ignoring the fact that it was stupid that I couldn't even remember my own name from my old life, or what my own face looked like, my canon knowledge was as up to date as it could be.

"Somehow, I am Tomura Shigaraki," I said to myself, incredulous at what I was seeing. It really was him- bright red eyes surrounded by wrinkled, pale skin on a face which looked like it had been through hell and back. It hadbeen through hell and back. I knew Tomura, more than I knew myself. This was trouble.

I was now the heir to the throne of All For One, the villain who could match All Might and plunge society into chaos. I was the gaming-obsessed successor who inherited All For One the Quirk, tore apart the world with a myriad of powers. I had become the boy who had destroyed his own family when the abuse by his father got too much, who had been groomed for years and years by a megalomaniac to tear society down. In canon, this manchild unleashed a Nomu on UA students, took on the Hero Killer, took down Re-Destro, earned the loyalty of the calamity Gigantomachia, a walking natural disaster.

I had become the grandchild of Nana Shimura, the seventh holder of One For All.

At that point, I paused. I had some massive issues on my hands, not least the use of my actual hands; I had rudimentary knowledge of how Decay worked in canon but no practical experience with it. But my issues extended further than this. I knew about One For All and All For One. I knew that Izuku was the heir. I knew about All Might and his limits, I knew where All For One would be defeated, and unlike the Shigaraki I had watched, I was conscious and fully aware of my Shimura heritage. Whether he had blocked it from his memory or whether All For One had manipulated him, the Shigaraki I knew wasn't aware of this.

This would be trouble for me going forward. I needed to work out where and when I was, fast, or some of my knowledge would backfire massively on me. And I needed to work out how I was going to blend in for the time being, or risk being absolutely destroyed by the single most powerful villain to ever walk the face of the earth for replacing his carefully-groomed future heir.

That thought made me pause. I don't know what I had done to deserve this; I don't think I was a fundamentally bad person in my former life to deserve being incarnated into the body of the future Grand Commander of the Paranormal Liberation Front. But it left me in a precarious position. In theory, I could have worked out where I was and run away, left behind All For One and the League and found my way to the Heroes. If All Might was in his prime, I could give him the means and information to beat All For One sooner. I could have turned Tomura back into Tenko, made him a force for good, redeemed himself and acted as I pleased to try to do the right thing.

Not knowing where I was limited me in any event, but at whatever stage in Tomura's life I had been spawned into (the gamer talk was infectious but effective) it was bad news for me. Before Kamino, and any acting out on my part would leave me the target of a walking demon in a black mask; no matter who I ran to, I knew so little of the world and had so little grasp on my own power that annihilation beckoned. After Kamino, and I was now in a position where any transgression would upset powerful people and bring down the whole house. If I suddenly made Tomura good and abandoned leadership, nobody would believe me on the Hero side, and even worse I could be hunted by League allies, PLF allies, even the Doctor. He could set Machia on me and the thought terrified me.

"You're a coward," I said to myself, gripping the tap carefully with all but one finger, "but you can't do anything else."

Self-justification had led to my conclusion. I was a coward for not deciding to change Tomura's course, but as much as my heart was screaming to try to do the right thing, every rational thought in my head and every last fibre of my survival instincts told me I could not. I would doom myself with very powerful people if I did not play the part, wherever I had been placed in this role. I needed more information, but more than anything I needed to be convincing, a brat, a danger. I needed to be Tomura.

I needed to do well.

I was so engrossed staring into the mirror that I didn't hear the door open, and didn't hear anything until the polite cough behind me. "Tomura Shigaraki... forgive me, but I have been sent to collect you."

That voice, deep and ethereal and oh so eloquent, told me all I needed to know. He wouldn't be around shortly after Kamino, which meant that so much of that was still to come in Tomura's life. Whatever happened, I knew I had an incredibly powerful ally standing right behind me in the room, and someone whose mission it was to protect me. So long as I didn't jeopardise that, I could rely on him and his powers.

I turned and nodded, and for the first time myself I took in the glory of the being before me. The tailored suit was immaculate, and the treacherous rational part of my brain wondered where it had come from, but there was nobody quite like him in the world. He was an eldritch creature of purple and black fog that billowed with every shift in the air like some malevolent flame, and for a moment I found myself marvelling at the piercing yellow eyes that glowed within the smoke, like some primal creature in the dark of night. "Kurogiri. I want to be left alone."

I wanted to be left alone to come to my senses more and take stock, but he didn't know that, and Kurogiri certainly didn't flinch at the deliberately bratty response from me. "I'm afraid I can't do that. Your presence to discuss matters is required, Tomura."

I made a show of grinding my teeth together in frustration. "Why? What's so important it can't wait for ten minutes?"

"He's in a more open mood than he was with you before." Kurogiri bowed. "He just wants to talk. He is open to your idea."

He? Oh no. Kurogiri didn't need to be specific for me to work out who he was. This was far too soon, and inwardly I tried not to scream at the timing. I needed days, weeks more time to get to grips with being Tomura, and now I had seconds before the most important person in his life came calling. Before I most likely exposed myself and was torn to atoms for taking his prized heir from him.

I sighed, and stuffed my hands in my pockets, determined not to touch the fabric of the hoodie with all five fingers. "Fine. Let's get this done."

I had never experienced it for myself, but in an instant Kurogiri's fog flared out from his head and hands and completely surrounded me, the whole world going dark and purple. There was a strange feeling, not unlike the moment on a fast rollercoaster where the force of launch knocks the wind out of your lungs, and suddenly we were in an all-too-familiar new venue, orange light glinting off of rows of glass and faded leather seating. I wasn't, surprisingly, at all dizzy, and that meant for a moment I could take in my surroundings.

The bar looked a lot nicer in person than it did in the show; I'd have drunk there in my old life. I don't know if Tomura drank in show, but if I survived what happened next I would be making the most of Kurogiri's drink menu. I would have earned it by that point.

As I came to my senses and stretched my arms, I heard the crackle of static and then froze. I thought that the show had exaggerated, but a wave of aura unlike anything I had ever felt washed over me from that television screen and paralysed me on the spot. This was raw, this was primal, fear and horror distilled and emanating from one screen set up on a table by the dartboard in the bar. There was only one person who could do this.

I did the only thing I could do and bowed my head, expectant and afraid. "Sensei? You... wanted to talk."

For a moment there was silence, and then a deep chuckle from the screen, a laugh that crawled up my spine and made whatever hairs remained on my deathly pale skin stand up on end. "I must say, I didn't expect you to be so willing to talk to me so soon, given your anger only an hour ago."

"Time can heal any wound," said Kurogiri sagely, and I wished that the warper was still standing behind me as some form of companion. The fact that he had moved behind the bar left me feeling naked and alone standing before the screen, and the ultimate evil on the other side of it.

"Quite so, Kurogiri," replied the voice on the screen. "And time also allows us a moment to think. I realise I may have been unduly harsh on you, Tomura, and so I wanted to give you another chance to convince me that your plans should be supported."

All For One chuckled again, and I felt it in my bones this time. "Well now, Tomura Shigaraki. Tell me once again why I should let you go ahead with your plan to attack UA High?"