Changing the past.

Something that might seem simple for those that can travel through time however turns out it is not as easy as it originally seems. Freezing time even truly stopping time to the point where you have to possess the correct skillset to even move properly let alone fight in that state. It turns out to be far easier than changing a past event from happening. This was something that Chloe Aubert learned the hard way when she attempted to change the past herself. Trying again and again in what seemed like an endless time loop to prevent one event from happening. One event that she knew if it never happened then things would have changed for the better because that event was the beginning of the end.

That event was the death Rimiru Tempest as he was seemingly fated to die after he managed to save her and the other from her class. She tried everything she could think of to keep him from dying. Do it over and over again...time after time...effort after effort...repeatedly trying to achieve something that from all angle seem to be something that just could not happen. Though like with a lot of things the answer to the question that she searched for was right under her nose the entire time.

Just do nothing

Do not attempt to do anything other than merely see him off as that is all it took to for him to live through what would have eventually led to his death. The things that sprang into motion did not seem that different at the start but the first change was made through his fight with Hinata unharmed and everything else was going hopefully down a much better path.

Though since there are people from another world that get summoned to this one then the reverse should be possible as well. Those summoned from this world to another world instead. It was this change that cause a delay as the monster that had been summoned from this world to another was not allowed to continue roaming freely. Since Chloe had succeeded in changing the past so that Rimiru does not die as he does originally. This set off a domino like effect that was changing the future not just of this world but of other worlds as well. Since the summoner that would have called back the monster that was accidentally sent to another world was not called back.

This monster having forgotten that they were, in fact, a monster was living in this other world as what was known as a D-rank villain and because they were never recalled at the time they were originally supposed to be before the past was changed. Said villain attempt to rob a bank only to get caught not once...not twice...but thrice. The hero that stops him has an encounter that changed not only their lives but this one encounter changed the lives of so many others. Just like how Rimiru surviving changed the world's future for the better this chance meeting did the same for that world as well.

This resulted in another unexpected change as there was another event that Chloe was not aware of because she was still sealed away as the sleeping hero at this point. An event where someone else was someone to were summoned to where her other self was sealed away at the time. They arrived but then having failed to manifest a Unique Skill and die. They were not like how Chloe was originally where she had failed to get one and then was on the course of dying. They just straight up died on the spot and vanished from existence afterward.

However, that hero that had an encounter that changed their lives and that of so many others for the better was All Might. The one he met that day was Izuku Midoriya. This was how the ninth user of One for All was chosen instead of going with the original plan that could have led to something unexpected because of what was still not known about One for All at the time.

Of course, Izuku was still summoned from his world to this one as a result of another summoner performing a summoning yet the location where he would appear was not in front of the summoner but instead where Chloe's older self was sealed away.

Izuku however still failed to obtain a Unique Skill of his own upon arriving in this world from his own and it was only made worse by what the world considered One for All to be in their terms. Izuku might have failed to have obtained and Unique Skill but One for All considered on par with an Ultimate Gift however thus that is what was considered.

Then there was another problem as he did not just have the mana of one person but including himself, he had the mana of nine people with seven of them having what the world considered Unique Skills that all merged into One for All. So Izuku's body was basically destroying itself however he was managing to keep it together. Izuku had experienced his body destroying itself fully before thanks to his fight with Overhaul though at that time Eri was with him. Her rewind quirk was reverse the damage he did to himself but even though she was not here now something was undoing the damage anyway. Having that experience Izuku was able to focus everything into his Full Cowling ability as he did not have time to think about why as he had to still be alive to think about why anything was happening.

The overflow of energy was stopping even as he reached 100 percent so he went beyond and continued to focus until he reached 1,000 percent. Even then that was not enough so he kept going to eventually reach 100,000 percent only to discover this still was not enough. Thus he kept going again until he finally reached 1,000,000 percent. Then he combined a 1,000,000 Full Cowling with his Delaware Detroit Smash towards what was directly below him. In the state Izuku was in, he was not caring what about his surrounding merely what was on his mind at that moment. That was unleashing all this power that he had managed to build up in hopes that it would stabilize his body after doing so.

The thing under him was the ark where Chloe's older self was sealed away though the blow was not enough to harm the ark itself it still was enough to blow everything else around him at this point. Izuku was channeling converting everything he could in power even things like mana which was not supposed to be possible for a human. Then he just vanished from view as when people came into check to see just what had happened through the ark was undamaged it seems like it was opened from the inside.