Tumblr prompt: If you're doing my prompts for bnha: OfA reached quirk singularity inside it's Eighth Holder, Yagi Toshinori, although it's full post-singularity glory was impeded by All Might's AfO induced injury.
When it's passed on to a lovely young, fresh, healthy and most importantly, malleable Ninth Holder aka Midoriya Izuku, it wastes no time mutating and going full eldritch nightmare fuel.
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"You might also get weak versions of the past users' quirks," said All Might, explaining One for All in more detail as Izuku worked at clearing the beach.
"R-really? Wow! But..." Izuku turned away from the pile of rusty corrugated metal sheets he'd been working on. "I've never seen you using something that could be another quirk?"
"I do try to be subtle about it, but it helps that they aren't terribly powerful or flashy quirks, for the most part. There's Float, my master's quirk, that I use to improve my hang time and speed, Smokescreen- which actually produces a kind of vapor- is good for hiding my transformation at night, Blackwhip helps me hang onto things, Danger Sense is obviously invaluable, and Fa Jin is good for an extra boost if I really need one. It's similar to One for All as a whole, but it duplicates and stockpiles energy short-term."
Izuku listened with wide eyes, watching raptly as All Might briefly demonstrated each quirk. "And... I'll get all of those? That's..." overwhelming, actually. An embarrassment of riches. To go from no quirk at all to seven, if he counted the ability to pass it on as a quirk in it's own right.
All Might chuckled. "Not right away. In fact, none of my predecessors seem to have gotten them. I think it's related to the length of time you have the quirk. I've held it for twenty years longer than the next longest." He went silent, contemplative, for a moment. "For me, they only started manifesting in the last fifteen years or so. Let me tell you, I was incredibly confused when Blackwhip first manifested. It, ahem, actually hurt a bit the first time. My skin wasn't used to it. Luckily, the whips were only an inch long that time... Not that I'm complaining, without the other quirks I wouldn't have gotten off so lightly." He patted the scar on his side, the remnants of a wound that had very nearly missed outright destroying several important organs.
Izuku was both obscurely disappointed and relieved that he probably wouldn't have to deal with new quirks suddenly manifesting until he was in his thirties.
"I'm just telling you now, so you won't be surprised. Especially if it does happen earlier. One for All becomes stronger with each wielder."
Izuku nodded and realized he'd just been standing and staring for the past several minutes. It was time to get back to work.
"Oh," said All Might, "speaking of the past wielders, One for All is just a little bit haunted."
Izuku dropped the metal sheet. "It's what?"
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Izuku spent the next several days convincing All Might that, yes, he still wanted to be his successor.
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It rapidly became apparent that whether or not a past user's quirk manifested and the strength at which they manifested had nothing to do with how long the person they were manifesting in had held One for All.
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"Well," said Present Mic. "I've seen a lot, but I've never seen an applicant suddenly turn into a mass of tentacles in the last minute of the exam before annihilating every remaining robot on the field. In all the testing centers."
He turned around. Everyone else in the room had been shocked speechless, and All Might in particular had gone as white as a sheet. Interesting.
"Anyone know what the little listener's quirk is registered as?"
"He doesn't have one," croaked Eraserhead.
"I'm sure he's just a late bloomer," said All Might, loudly, before standing up and rushing from the room.
Okay, what?
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"My boy, are you sure you want to register your quirk as 'Eldritch Abomination?'"
"I mean," said Izuku, trying to pick an egg up with Blackwhip without crushing it to bits, "I'm not sure what else to call it that would encompass all of the different aspects." He crushed the egg and sighed heavily. "Like you said, people thinking I have multiple quirks could be bad."
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All for One reviewed the stolen footage of the UA entrance exam for the tenth time. As before, what he saw was undeniable. That student, so prominent in the last minute, was using multiple quirks. More importantly, multiple quirks he was intimately familiar with. One for All had a new host.
"Kurogiri," he said, "make arrangements for me to leave Japan."
"Shigaraki Tomura as well?"
"Oh, no," said All for One, pleasantly, "he'll still carry out my initial plans. I just don't want to be in the country when he does it." Or, more accurately, near the monstrosity One for All had become.
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One for All blinked awake in the bedroom of their ninth member and smiled beatifically. They had a job to do, and this time they had all the tools they needed to do it.