Disclaimer: Hunter x Hunter characters & places belong to Yoshihiro Togashi (hopefully I spelled the name right) and My Hero Academia characters & places belong to Kohei Horikoshi (I'm bad at names, please tell me if I spelled either of those wrong). Some plot elements may be taken from MHA, but the story itself belongs to MarkTwainTwo (hi!).

Note: Okay so I have seen so many HxH and MHA crossover stories, whether Killua or Gon or someone else gets thrown into the MHA world. Some of them are good, and some of them are... *cough* yeah, I won't finish that. I can't say mine will be any better, but I thought I'd give it a try.

You can read the following, or just skip it, but it might explain my reasoning behind some stuff, soo...

- Now, some crossovers have Killua and/or Gon having less power than the MHA characters, and some have them super overpowered. Tbh, I don't think the OP thing is all that unrealistic (I mean, have you seen what those kids do in the HxH universe?), so I'm leaning toward making them stronger than the MHA characters. Killua is literally an assassin who has killed a whole bunch of people. Both of them fought Chimera Ants and lived and won against a bunch of them. They're both basically superhuman even without Nen, so even if the MHA characters have abilities similar to Nen, Killua and Gon are basically on their level without the Nen, let alone with.

- Gon's an interesting and funny character, but my favorite is Killua, so guess who gets to be tossed into the MHA universe first?

- I know a lot of people ship Gon and Killua, and I don't have anything against LGBTQ, but 1) I don't like writing romance in general, and 2) I think they're like brothers, so I'm going to leave romance out of this.

- A bunch of stories toss Gon and/or Killua into the MHA universe and have them be like "huhh? What's a quirk?" and honestly I don't think that's realistic at all. They might not know MHA's interpretation of the word "quirk" but it does actually mean something! At the very least, Killua would assume they're asking about "quirk" as in "peculiar behavioral habit." He'd probably even realize that there are people with powers here and it doesn't look like any place he knows so obviously, they're referring to their abilities as quirks. Gon and Killua (or at least Killua, dunno about Gon) aren't that stupid and "Quirk" isn't that weird a name.

- I'm not entirely certain how old Gon and Killua are after the end of the 2011 HxH series, but I'm gonna pretend they're 13. Why? Because.

For both my own reference and that of anyone who forgets what the hell all those Ten and Hatsu and other principles of Nen even are:

Ten: keeping aura nodes open but the aura around the body instead of away from it.

Zetsu: stops the aura coming from the body altogether, which both hides the user's presence and makes it easier to sense the presence of others, but also makes the user more susceptible to Nen attacks.

Ren: offensive counterpart of ten, huge amount of aura, forcing aura onto someone else (referred to as bloodlust).

Hatsu: carries out specific functions of Nen that are usually unique to the user (i.e. Gon's Jajanken and Killua's Godspeed).

Gyo: focusing aura into a specific part of the body (usually used on the eyes to see other peoples' Nen).

In: similar to Zetsu in that in conceals presence, but it does so by hiding and not stopping aura flow.

En: extending Ren and using Ten to feel the shape and movement of anything in the area En covers.

Shu: shrouding an object in aura flow.

Ko: similar to Gyo, but it limits the aura to that specific part of the body, making that part very strong but the rest completely unprotected.

Ken: using Ren for a long time as a shield.

Ryu: distributing different amounts of aura to different parts of the body.

Okay, I think that's all. Onward!

Killua

The stupid portal was by no means the worst part of his day, although it was rather annoying. Ranked, the three worst parts of his day were:

1) The fact that he'd already eaten all the chocolate robots at the nearest candy store.

2) The fact that he'd already eaten all the rest of the chocolate at the nearest candy store.

3) The fact that he'd already eaten all the chocolate within a 100-mile radius of the hotel he and Alluka and Nanika were staying in.

In fact, the portal probably was ranked in the fifties. Compared to the rest of his week (including Kurapika's distress call, which turned out to be a false alarm but was still nerve-racking, and Illumi nearly catching up to him and Alluka in York New City), the portal probably wouldn't even deserve mention... if it hadn't been for one tiny little fact.

Killua fell into the portal, and Alluka hadn't.

Now, he was falling through the sky in an as-yet-undetermined part of the world, without his practically-defenseless little sister that he'd sworn to protect.

Killua thought maybe he should make a list of the things he had to complain about for when he met up with Gon again. They'd split up earlier that year- Gon with his father and Killua with Alluka -promising to meet at Heaven's Arena in six months' time. He'd see Gon again in only two months, and he wanted to be ready to rant about all the annoying and boring things that had happened in the last four months. Like falling through a portal and then through the sky.

Oh right, he was falling through the sky.

He heaved a sigh, stuck his hands in his pockets, and righted himself so he'd hit the ground feet-first. He eyed the fifty-story apartment buildings as he free-fell past their top floors... but no, he might as well just hit the ground. Suddenly falling onto someone's balcony was sure to create a fuss. Luckily, it was evening, so there were fewer people to accidentally hit on the street below.

Twenty stories left...

His En was still terrible (okay, sue him for not practicing while he was worried to death over Gon and Alluka), but he could still sense a presence when someone wasn't even trying to hide it. So he tilted his head a little and eyed the top of some building as he passed it, noticing the man standing there.

Ten stories left...

Wait- did he have, like, eight extra hands? The silver-haired assassin scrunched his nose, trying to figure out why there was a hand on the guy's face and how he had extra hands seemingly without using any Nen.

A second person was staring at him from the shadows- someone he couldn't make out with his eyes. There were more in the building (and the buildings around him), but honestly, Killua didn't care. Either they were as weak as their auras indicated, or they were so strong that they could hide it and then he'd be dead anyway and it wouldn't matter.

Two stories left...

Killua forgot about Extra-Hands, and concentrated Ryu to bring 80% of his defensive Ken to his feet and legs.

One story left...

Then he hit.

Shigaraki

When All for One contacted them to say he'd used a rare one-time-use quirk to summon a powerful being, Shigaraki expected someone... well, powerful.

Not a freaking ten-year-old boy!

Of course, when the portal first opened in the sky, it was too high up for Shigaraki to actually see the boy. He'd got a glimpse of white hair as the figure tumbled through the air. As it got closer, he saw it was a very small figure. It looked like a boy, who proceeded to stick his hands in his pockets and not scream. Screaming and flailing would make sense for a little boy, but when the white-haired child fell past Shigaraki's roof, he glanced over at Shigaraki... and his face was so bored. As if falling hundreds of feet was an everyday occurrence that didn't deserve a second thought.

So maybe the child was powerful?

Shigaraki turned to Kurogiri, who was in the shadows behind him, and opened his mouth to ask the other man to portal the boy to their roof before he went splat.

Then the ground exploded.

Okay, not really, but it very well could have from the noise and debris it created. There was a BOOM as the kid hit the ground, and pieces of sidewalk and road flew up, some even reaching Shigaraki's roof. His eyes widened, and he darted over to the edge to see what had become of the kid.

There was a... crater.

No kid. But a crater. A crater that was at least ten feet deep, with two footprints in the middle that were another couple of inches into the concrete.

That should have shattered the kid's legs, if not the entire kid.

Kurogiri hurried up next to him and sucked in a breath. "Oh fu-"

"Yo."

"-fudge," Kurogiri ended in a squeak, and both of them wheeled around to find the kid, completely uninjured, standing behind them with hands in pockets.

The kid's eyebrow rose. "Yeah, I've heard way worse swear words, you don't need to turn them into food. Fudge is good, don't insult it." And he honest-to-gosh stuck out his tounge.

He really was a kid.

Shigaraki was so startled by the chain of events that words failed him. Luckily, Kurogiri's brain still worked enough for him to cough and say, "What's your name, kid?"

The boy's dark sapphire eyes flicked lazily over the two adults in front of him. "You can call me Killua. And unless you can offer me information, I have no interest in who you are. So I'll give you exactly one chance to live." All of a sudden, both Kurogiri and Shigaraki were nearly frozen in place by the waves of bloodlust emanating from the kid- from Killua. "Tell me where I am, and what that portal was, and maybe I won't kill you."

The bloodlust retreated just enough for Shigaraki to choke out, "Musutafu. You're in Musutafu, Japan. The portal was Master's. He wanted someone with a strong Quirk to battle the heroes-"

And the killing intent was suddenly just... gone. Shigaraki blinked. He'd never witnessed so much bloodlust just up and vanish like that, like a switch had been thrown.

"What the hell?" Killua muttered at his shoes. He glanced back at Shigaraki. "Who's your Master?"

Shigaraki stood taller. "The greatest villain, All for One! He-"

He didn't get any further. Killua snorted. "That doesn't tell me anything, old man." Shigaraki spluttered. Old man? "You could be speaking another language for all I got from that." He narrowed his eyes. "Hmm... this "All for One"... can I speak to him?"

Shigaraki started to shake his head, trying to find the words to say "no because I'm the League of Villains leader that everyone actually sees and Master only talks to us through a TV most of the time, so you have to talk to me and we want you to join the LoV and fight for us" in a concise manner.

Before he could come up with something, Killua gave an aggrieved sigh. "Well, I guess I won't kill you, 'cause you did tell me where I was. But if you come after me in the future..."

Kurogiri unfroze again and stretched out a hand, trying to prevent Killua from leaving by manifesting a portal beneath him. But the snow-haired boy had already vanished, leaving his unfinished threat hanging in the air.

"...Well..." Dabi said from behind them. "That was... interesting."

Note: Just to be clear, Killua isn't actually ten, he's 13. Shigaraki's ten-year-old comment was an exaggeration on Killua's age. Just to be clear.