Shortly after the fight against Overhaul, Deku gets asked by someone to help save people. He accepts, not fully knowing what he is getting into.
He feels as if he were dying. Fading in and out of consciousness, he can hear someone calling his name. 'Is Eri alright?', he questioned to himself blearily. He tries to look, open his eyes, but his body won't respond. As he lay there, broken and bleeding, he wonders if he could've done a better job if he was stronger. Would Mirio have lost his quirk? Would Sir have had to fight against Chisaki? Would Eri have come back after Mirio took her away? He doesn't know. He just wanted to help, to save them.
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He can't feel anything anymore. 'Am I dead? I feel as if I am just floating in nothing.' he tries to move his arms, legs, head, eyes, yet nothing responds. 'Would my body even exist if I am dead?', he questions. He feels more sure of his hypothesis of him being dead. Realizing it as an exercise in futility, he gives up on trying to move.
"Correct. You have no body as of this moment." a chipper sounding voice says to him, seemingly from his very mind. 'Makes sense. Now, if only I could figure out what to... Wait. That wasn't my voice. Is this purgatory? Am I going to be judged and sent to where I belong?' he tries to work out where he was and who that voice was, only to realize that he's far too calm. "Ah. It seems you've noticed. No, this is not purgatory. No, I am not your 'Judge'. Yes, you are far too calm but that is normal. You don't actually have a body, that includes the brain. It makes dealing with spirit bodies, or astral projections of the consciousness, much easier. It's best not to dwell on it. Now, before we continue, I want to clear something up. You are not dead. I am not some 'god'. Your body is still alive, barely, but your consciousness, soul, spirit, is here in this space with me."
Midoriya listened intently to what the voice was saying. 'If I am not dead, then what am I doing here? Clearly, this person was the one who brought me here, but why? Does this person need me for something?' he thought slowly coming to a realization. "Good. You've almost completely guessed my intentions in bringing you here." the voice confirmed his suspicions.
"This is going to be a bit long, so relax Midoriya." his voice taking a more serious tone. "I need your help as you've already surmised. My job is to make sure that the timelines of a few hundred dimensions stay relatively on track. The problem is that one of my 'co-workers' is messing around and changing the dimensions under my purview. It's causing some of them to go so far off track that if the higher-ups were to catch wind of them, they'd 'scrap' 'em so to speak." his voice gains a saddened note that shows how upset he is. "I don't like it. Sadly, my abilities can only fix minor differences in a timeline without being there physically, and due to restrictions on my job, I can't leave here. What I need is for you to drop in to one of these timelines, and remove the problem. Of course, I'll lend you something that will make it a bit easier and even help locate the anomaly. Once you complete this task, I'll slip you back in right when you left along with a little gift. If you refuse or fail in this job, I will still put you back in you back into your body but I'll remove all knowledge of this meeting. Knowing your heroic tendencies though, you'll save them, right Deku?" his voice regains it's chipper and happy tone nearing the end.
'Why does it feel like he's not telling me everything? Though it's obvious I'll help, I wouldn't be able to call myself a hero otherwise. It's a bit unnerving not really feeling the emotion behind that thought and only realizing I should feel and think that way based on my memories.' he thought all too calmly. "Fantastic! Now I need you to not struggle as best as you can. I have to insert you into a newly created body. I won't lie, it's going to be a very uncomfortable process. But don't you worry! It shouldn't be painful and the new body looks identical to how you actually look."
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He feels someone or something grab 'him', very quickly he realized what the voice meant by 'uncomfortable'. It felt as if he was on the world's fastest and jerkiest roller coaster, not even knowing which way was up and being thrown in every conceivable direction. Then he felt his entire being compressed and stuck through something that felt like a straw five sizes too small. It was as if a child was trying to push a square block into a hole meant for a circular one. All of sudden, he could feel again. He could feel the heat of the sun, the wind caressing his skin, his back laying on some grass. He quickly rolls over and up onto his hands and knees, retching while his stomach was doing back-flips.
After thoroughly emptying his stomach he started frantically questioning himself and his sanity, "Oh my god, what have I agreed to!? I don't even know his name! He didn't even really give me concrete details! What if he was lying? Why did I trust him? Was any of it even true!?" he quickly said mumbling almost incoherently to himself.
After calming a bit he realized why the being didn't have him in a physical body when talking to him. His emotions would've made him hesitate in undertaking the task. He started thinking about his parents, All Might, his classmates, Eri, Mirio, Sir...
"I hope they're okay. Alright, I need to focus. I should be in a different world if he is to be believed. Will I even understand the language? I somehow doubt they are going to speak Japanese. For now, I should figure out what I should do. Eventually, I'll need to locate this 'anomaly' and remove it. It sounded like he meant some sort of object, but I can't be sure. Wait, didn't he say that he would lend me something to help?" he rummages through his clothing quickly noting that he is dressed in an unfamiliar uniform. "...nothing? Oh no, was he really lying from the beginning!?" he twists his body around looking for something, anything, that seemed like what the being was talking about.
*DING*
He came to a stop, feeling uneasy about that noise that seemed to come from nowhere and everywhere. He got into a fighting stance, looking around with an eye far sharper than earlier. While channeling One For All into his limbs, he slowly checked his surroundings once more. Then he heard someone speak. No, not someone, something.
*Host, congratulations! You are now the proud user of the Nasuverse System!*
Midoriya Izuku, 15 years old, hero-in-training, was completely speechless.