Intro

Hello, and wow, this is a brand new one…

I haven't seen this idea done all that much, and I really like both series, so why not give it a go? So here we are, Ben 10 x Touhou Project – "Transdimensional Watched Child" to keep in-line with ZUN's naming convention for his Windows Touhou games :P

But before we get into the first chapter, a few quick things:

This Ben Tennyson is from the classic series/original show, meaning that he's still 10 years old and has only unlocked the aliens (and MAYBE more later on) from what we've seen in the show (and animated movie, "Secret of the Omnitrix").

They include the original 10 (minus Ghostfreak), Cannonbolt, Wildvine, Upchuck, Ditto, and Way Big. (Blitzwolfer, Snare-Oh, Frankenstrike and Zs'Skayr were mainly used in one-off episodes, minus Zs'Skayr, and won't be used. Way Big is the only exception)

However, this fanfiction takes place DURING the episode "Back with a Vengeance" and for Gensokyo, pre-EOSD, so the "Master Control" (will be explained later for the Touhou readers) is what caused these aliens to become unlocked despite some not being unlocked at that point yet.

And if you come from one side of the fandom and don't know about the other, I'll try my best to explain everything within the story, and if you still don't understand, I've got a glossary at the end 😉

My first fanfic on this site, so let's hope it goes smoothly with you lot 😊

So, enough of that, let's go!

Chapter 1 – "And Then There Were Youkai"

Ben Tennyson, a 10 year old boy with brown hair and a signature white t-shirt with a black stripe down the middle, had stumbled across the "Omnitrix" on his first day of summer vacation, a black and grey wristwatch with white stripes on both sides of it, from outer space of unknown origin (for now), that could allow him to transform into any alien species from the set of 10 that the Omnitrix allowed him.

He, alongside his Grandfather (and recently revealed "Space Police" Plumber), Max Tennyson, as well as Ben's cousin Gwen, travelled across America to defeat evil doers and at least attempt to try and enjoy a normal summer vacation when the "villain of the week" was nowhere to be wreaking havoc.

Today was a relatively carefree day at Niagara Falls, with Ben messing with the Omnitrix randomly out of boredom to the point of accidentally unlocking the "Master Control", as he dubbed it. With it, it removed all of the limitations that the Omnitrix had before its unlocking, such as being forced back into his human form after 10 minutes of being an alien to "recharge", as well as transforming him into the wrong alien on a few occasions despite choosing someone else.

It meant unlimited transformation time, being able to switch in-between aliens with just a thought and having access to every single alien in the Omnitrix's database – over a million, though Ben didn't know this one in particular.

It was a really fun day, pulling pranks on his "dweeb" cousin Gwen and abusing his powers such as using his flight alien (Stinkfly) to give rides to tourists and make some cash on the side.

Suddenly, his intimidating and ruthless arch-enemy Vilgax, a squid-like but intimidating and powerful alien that had conquered 10 worlds, had teamed up with Ben's other prominent enemy – Kevin Levin, a human who had mutated and had become a chaotic amalgam of all 10 of Ben's aliens at the time and dubbed himself "Kevin 11", having all of Ben's powers plus his own.

After an encounter with the Tennyson and the Omnitrix's protective feedback blast after he tried to forcibly remove the Omnitrix from him – he gained access to all of Ben's alien's abilities, albeit a tenth as strong as his individual aliens aside from physical strength, which seemed to surpass that of even Ben's strongest alien at the time, Four Arms.

Both Vilgax and Kevin had come to an agreement, after a short brawl with each other in space, to defeat Ben together albeit under different motivations - Vilgax wanted to finally claim the Omnitrix for himself, whilst Kevin just simply wanted revenge on Ben after his accidental mutation.

Vilgax and Kevin had assaulted Ben, Gwen and Grandpa Max out of nowhere nearby Niagara Falls, but through unlocking the Master Control, had a much easier time getting Max and Gwen to safety whilst simultaneously battling the once defeated foes through constantly switching aliens during the battle to control the fight in his favour, which was undoubtedly working for him.

Now we have arrived at the centre of a massive bridge over a ton of water below, with Vilgax and Kevin standing alongside each other side by side, and with Ben, Gwen and Max on the other alongside Max's RV and "Motor Home", the Rust Bucket.

"When I obtain the Omnitrix… I shall luxuriate in tearing you apart… TENNYSON." Vilgax screeched out to his former arch enemy in his usual low-pitched and slightly distorted voice.

"Heh, why wait?" Max replies calmly, readying a gun behind his back.

"Come and get me." He replies again to himself stating, finally revealing the gun that he had in his possession from his "plumber days", the Null Void Projector. It was a gun that could shoot a yellow portal that could suck in and transport whoever entered into the Null Void, a hellish alternate dimension where the universe's worst criminals were banished, also operating as a gigantic dimensional prison.

The portal then opened around 20 meters or so behind the evil duo, already proceeding to pull them in.

"I've got it, Grandpa!" Ben shouted in his "Diamondhead" form, a Petrosapien that was extremely durable and could form diamond structures and projectiles, as he ran towards the two out of cockiness over his newfound Master Control.

"BEN! STAY BACK!" Max shouted, but to no avail as Ben kept running towards the two anyway, who weren't making any progress in attempting to outpace the sucking of the portal.

"Nnnnnrgh! What's happening?!" Kevin shouted as he was the first to have his body give up on him as he flew into the portal.

"The Null Void…! NO!" Vilgax also shouted as he got pulled in as well, with Diamondhead stopping closer to the portal to investigate and reflect on what just happened.

However, it was a bad decision, as Vilgax's claws latched onto the sides of his face from the portal and pulled him in with him, forcing the portal to close.

"BEEEEEEN!" Gwen and Max screamed in utter shock and horror.

The Null Void?

Soon enough, a yellow portal opened slightly above the ground and spat Diamondhead stomach-first to said ground, with no Kevin or Vilgax in sight.

Diamondhead then slowly tried getting up after observing his surroundings a bit, becoming extremely confused.

"… Uh, is this what squidface (Vilgax) called 'The Null Void'?" Diamondhead said to himself, shaking his head a bit and rubbing his eyes as he stared into the environment.

"I'm in nothing but some crummy forest!" He announced to himself out loud.

He then quickly shaped his hands into sharp diamond blades, after just remembering that Kevin and Vilgax got sucked in along with him, but…

"Huh? Where did they go?!" He once again said out loud, unable to comprehend why Vilgax and Kevin weren't where he was considering they got sucked into the same portal, before reshaping the blades into his hands again.

The Omnitrix symbol on his chest flashed red without any warning sound this time and within just a short second, transforming him back to normal immediately… Something that was extremely weird to happen.

"Aw… Man…" Ben whispered to himself as he was suddenly unable to stand up and fell to the floor stomach-first, getting knocked out as he lay on the wavy dry grass below him.

The Omnitrix then flashed in inverted colours for a split-second multiple times, like it did when he first unlocked the Master Control, as it beeped away and cycled through all of his aliens yet again.

…

As Ben's eyes started opening, he was greeted with a lot of birds chirping as they flew from tree to tree as well as the sun shining on him through one little area where the trees hadn't fully blocked it.

"Aw man, I'm up! I'm up!" Ben shouted as he fully awakened to still find himself in a peaceful forest, this couldn't be the so-called "Null Void" at all – can't it?

As Ben struggled to his feet, he almost immediately took a look at the Omnitrix on his left wrist, which was fortunately still on, so Vilgax and Kevin didn't find him yet if they were indeed teleported to different positions. It was in "Active Mode", so the hourglass symbol was green and ready to be used, but something wasn't right, as our young Tennyson will soon discover.

Wanting to take a look around, Ben touched the button on the Omnitrix for the core to spring up, with the hourglass transforming into a Rhombus, showing all of his aliens as silhouettes as he turned the dial to find XLR8, his Kineceleran form that was all about speed… Or was "all of his aliens" the right description?

"What the… Why do I only have 3 of my aliens?!" Ben shouted at himself, as he kept turning the dial angrily to only discover that the only 3 of these aliens that he has were Four Arms, Heatblast and Grey Matter… Not good.

"Aw man!" Ben shouted again as he facepalmed himself hard.

He could then hear a large amount of rustling in the trees, though much fiercer than usual… Someone was here… Not good, AGAIN.

"Well, well, well, it's been a while since a human has come down this way, I'm hungry~" A voice said, spooking Ben a bit.

"Who's there?!" Ben announced, having his right hand already on the Omnitrix.

The voice then jumped down, and from Ben's perspective above him and most likely from some tree, revealing itself to be…

"A girl, huuuuuh?" Ben said out loud, surprised at what he was seeing, nothing like he's ever seen in his own world. She appeared human, but her features and appearance made it clear she wasn't an ordinary passer-by.

"A child too, yuuuuum…" She said, licking her lips.

"This girl is an utter freak! Does this even sound lega-" Ben thought to himself in his mind, cut off with her next chilling words.

"You will make for great food; I've been hungry for some time… Hehehe…" She replied, having the evillest smile Ben has ever seen, even more so than Kevin or any other one of his foes back home.

"Sorry child, but when humans come down this way, Youkai aren't really known to just let them cross without being devoured first~" She stated as she excitedly moved closer, though slowly.

Ben started walking backwards, but wasn't too scared aside from his initial reaction.

"Heh, I'll give you a knuckle sandwich, four of them to be exact!" Ben announced as he tapped the button once more to bring up the core yet again, cycling through his other 2 aliens until he landed on Four Arms.

"Want to play with your little toy before you go? Okay, I'll wait a bit~" She said as she sat down and rested her hands to the both sides of her face, staring with most interest.

Ben then smirked as he raised his hand extremely high and slammed down on the watch, blinding the Youkai as a green light erupted from Ben as he started to have his DNA overwritten and changed.

When the light died down, she looked forward after she stopped covering her eyes because of the light in utter shock.

"Wha-What the?!" The youkai shouted, staring forward in utter disbelief.

Ben then stared at himself in his new alien form, but he didn't have four arms, only two, and they weren't claws either…

"Heatblast? What is this, a running gag?!" Heatblast stated in utter annoyance, now being a Pyronite – an alien species that thrives on Pyrokinesis and is made up of red rocks and lava, and was actually the first alien Ben ever transformed into when he first got the watch… And here as well in this new place.

"Not cool, you stupid watch! And I don't just mean that literally!" Heatblast stated as he finally returned his intention to the Youkai, whom went from an evil smirk to an extremely frustrated and angry frown.

"What the… So, you're not a human then?!" The youkai shouted for a response, getting increasingly frustrated with each passing second.

"No, I'm a human who doesn't have any time for ya'." Heatblast replied simply, flaring up his palms as he stretched them towards the ground.

"Grrrr…! WHY YOU LITTLE!" The youkai shouted as she foolishly jumped towards him, but in the absolute worst way possible with Heatblast shooting two flame beams from his hands which propelled him upwards at a fast rate, with the Youkai now gliding towards the tree that was once behind him, banging her head and falling down in the most cartoonish way possible.

"I… hate that kid." The youkai whispered to herself before she became unconscious.

Meanwhile, Heatblast glided above the forest, where he could see the entire area, he was now staring at a massive forest that stretched extremely far, but there were some notable places.

He could see for one that there seemed to be this Shrine-like area in the corner of his eye on his right, though quite distant, and also what appears to be a village that's barely visible up ahead, and even further to his upper left was some sort of dark mansion in the middle of a massive sea?

"Aw, man…" Heatblast grumbled to himself squinting in annoyance as he hovered over the forest.

"Alright, better hope this watch hasn't affected my luck either." Heatblast replied back to himself, choosing the shrine area as the better place to go to, maybe there was someone there? It was much closer than the village, and let's not even get started on said mansion.

He then concentrated his palms behind his backs and propelled himself forward towards the shrine, whilst fighting the resistance of the wind which blowed with a warm breeze towards him, he gave himself time to ponder.

"Hold on, if Grandpa sent us all into the 'Null Void', wouldn't he try getting me out? But if that didn't happen already, then am I not in it?" He wondered to himself.

"Gimme a break, now I gotta find my way outta here…"

The shrine was now fast approaching Heatblast as he continued gliding towards it, but not for much longer.

The Omnitrix symbol on his chest started flashing red alongside beeping the infamous beeping tone… He was about to transform back.

"Aw man! I HATE that sound!" Heatblast shouted to it as a red flash consumed him, followed by a now higher pitched screaming sound following out of it.

"Aaaaaaaaaaaah!" Ben screamed as he was falling right towards the shrine. Luckily, of course, the top of the shrine roof wasn't all too far from where he was gliding since he was descending a bit, but this was still gonna hurt.

He then impacted the shrine roof, spitting out saliva as he landed on top.

Groaning from the impact, he tried pushing himself up only to slip and roll from the roof to the ground, which hurt like hell.

He then finally got up behind the shrine and stared maliciously at the Omnitrix, which was now timed out as indicated by the red hourglass.

"We're gonna have a serious talk when we get back, you've gotta be kidding me on that one! I've never turned back that quickly before!" Ben screamed at it when he started looking behind him at the big shrine.

"Woah… What is this place?" He asked himself before walking around it, seeing up ahead what appeared to be stairs that seemed to go down, quite a way down too.

"No, seriously, someone has gotta tell me what's going on here." Ben asked himself, sort of expecting an answer.

"Then allow me to make my entrance." A British voice said, completely unfamiliar to Ben.

He then appeared from a blue portal, making his appearance with a white lab coat, grey eyes and black hair.

"Waaaaaaaaah!" Ben shouted as he jumped back and tried pressing the button on his Omnitrix, though to no avail as it was timed out and responding with a beeping noise that descended in tone.

"Please, do not worry Benjamin, I am just simply here to speak to you." He said calmly, giving a small smile.

"Who are you?! Why do you know my name?!" He asked frantically, trying to find a way to get out of the situation, whilst tapping extremely hard on the Omnitrix's faceplate multiple times.

"I am Professor Paradox, and we have met several times before, Benjamin." He replied, still retaining his smile.

Ben then slowly lowered his Omnitrix arm and stared at him all confused, whilst still squinting in aggression as he crossed his arms.

"Met you? I've never seen you in my whole life! I bet you might be one of Grandpa's friends…" He replied, trying to think.

"Haha, I am as well… But I was more or less referring to the you that I would first meet 5 years from now." Paradox stated, which made Ben's eyes widen.

"The me from 5 years now?" Ben asked, confused.

Ben just couldn't take this seriously.

"So, what are you then, some sort of time traveller?" He asked.

"Still clever, even before you have matured…" Paradox replied back.

"HEEEY!" Ben announced as he took offense to that.

Both of them then heard loud footsteps echoing off of the bottom of the distant stairs, and was somehow really audible.

"Please listen to what I have to say next, I do not have much time, but the reason why you are not in the Null Void is simply due to a higher force." - P

"Wait, I'm not? And a higher force?" – B

"Why, yes… Whilst I fear she may even be watching us at this moment, I must stress how important it is that you find her, and develop allies in this journey that you now find yourself in. You were not ever supposed to come here, and this has now marked the beginning of a new timeline that has split off from your main one." - P

"Woah, wait… Timelines? Main one? Her? You're not explaining a whole lot." – B

The footsteps were getting a lot louder as this person was ascending up the stairs.

"Because these are details you will discover for yourself, but do not worry Benjamin, this will not be the final time we meet." – P

"So, wait, if you can time travel, did the new Sumo Slammers game do well? Is it worth it? Do we have jetpacks? Do you know what happened to the Master Control?" Ben asked as he pointed towards his Omnitrix.

"Haha, all in due time, Benjamin. As for your 'Master Control', I am afraid it's gone, for now anyway." – P

"Huh? What's that supposed to mean?" - B

"Your Omnitrix was reset when you were thrown into this new dimension, the Omnitrix was not able to establish a good enough connection to the Codon Stream, and therefore had to reset in order to reobtain a good signal." – P

"The Codon—Augh, whatever. But can't you take me back with you?" – B

The footsteps were getting extremely loud now, whoever it was, was nearly up top.

"I am out of time, but just know that the one who sent you here is keeping you and the others here for some purpose… I cannot get you out, but I promise I will explain a bit more to you when the time is right." – P

"Farewell for now, Ben Tennyson, good luck!" Paradox stated as he vanished through the same blue light.

"Aw man! And 'others'…?" Ben whispered to himself as he quickly went out of sight from the front of the shrine by going back behind it.

Ben could then hear talking in the distance, as the person who was ascending up the stairs was finally visible as Ben peeked around the corner… In fact, it wasn't just one person, it was two of them!

One of them appeared to be some sort of Shrine Maiden as evident by her outfit, with a big red bow behind her brown hair, whilst the other had a huge witch hat and get-up and had blonde hair with this big grin.

"So Reimu, I bet you're dying for some excitement around here, eh da ze~?" The blonde girl asked, still retaining her grin.

The shrine maiden sighed heavily as she shut her eyelids and simply replied back "I'm dying financially already; I don't need myself to go along with that..."

Ben then continued watching the girls getting closer to the shrine, wondering on what to do.

"Honestly, I'd rather these incidents just cease already, it's too much effort." The shrine maiden girl now identified as Reimu said.

"Aw c'mon, da ze! You need a bit of excitement in your life!" The blonde girl announced, with Reimu just dismissing it with another sigh.

"If said excitement was easy and got me some money, Marisa, I guess I'd be fine with it." Reimu replied to the also identified Marisa.

Reimu then walked up to the front of the shrine, opening a small box that Ben only just briefly saw for a few seconds when he was gliding down as Heatblast earlier… Or at least that's what it sounded like to him, as he couldn't really see them anymore and could only hear what they were doing and saying.

"Yeah, still nothing." Reimu said to herself as she sighed once more.

Reimu then sat down on the floor as Marisa sat next to her.

"Also, by the way da ze, did you smell something off on the way here? Kinda smelt like smoke." Marisa asked, with Ben facepalming himself as he knew that was all from him as Heatblast.

"I don't know, maybe, but it's probably nothing." Reimu replied as she looked ahead with the beautiful scenery and sunny sky.

Ben's Omnitrix hourglass then turned green as it made a large sound indicating that it was ready to go and had finally recharged.

"Aw, maaaaaan!" Ben said to himself in his head.

"What was that, ze?" Marisa asked with a concerned tone, with Reimu and Marisa standing up.

"Sounded like it came behind the shrine. You think it's someone coming to make an offering?" Marisa asked, whilst Reimu just shrugged.

"I don't know." She simply replied, walking with Marisa to the side of the shrine.

"Oh man, oh man, oh man! What am I gonna do?!" Ben asked himself in his head once more, trying to think of a way out.

"Hey, anyone there?" Marisa asked out loud.

Ben then pressed the button on the Omnitrix, and for once, he really wanted Grey Matter.

He then cycled through Heatblast and Four Arms until he finally landed at the Grey Matter silhouette, with Ben just about to slam down on his watch.

"What are you doing, da ze?" Marisa asked as she and Reimu stared at him with a confused look on their face, whilst Ben looked at them in utter horror… Caught.

*Chapter End*

Pheeewie, long one. I hope this was a good introduction into the cross weaving of Ben and Gensokyo, and I also hope you lot enjoyed this chapter.

Reviews are greatly appreciated, even critical ones to help me improve the story, and you can also feel free to ask me questions in them!

Here's a glossary for those from either the Ben 10 side or the Touhou side and haven't got much of a clue as to what specific terms or characters are yet. Even if I did explain it before, I'll put them here again as a reminder:

Ben 10

Ben Tennyson – The main protagonist of the entire series, and was once an ordinary 10 year old boy. He was unpopular with his classmates and got bullied before going on Summer Vacation with his Grandfather and cousin, he stumbled across the Omnitrix on that first day of vacation and used it to save the world countless times alongside his already mentioned Grandfather Max and cousin Gwen.

Vilgax – An evil and intimidating alien warlord that looks similar to a squid, and has conquered 10 worlds in his lifetime. He devoted his life into obtaining the Omnitrix, to use on his blank DNA army and synchronise them with the watch, creating an entire army of Omnitrix wearers to dominate the universe.

Kevin Levin (Kevin 11) – An 11-year-old kid that met Ben in New York, and was capable since birth of absorbing energy with his palms, and even became friends with him temporarily. After a disagreement with Ben on morals, he had tried absorbing Ben's aliens and using their powers to defeat him, which ended in failure as he tried forcefully removing the Omnitrix, which responded by pushing him away with a large feedback blast to protect Ben. Following this, the DNA he absorbed from that blast later went out of control in him, and caused him to mutate into an amalgam of all 10 of Ben's aliens. This, coupled with still being able to absorb energy, made him "Kevin 11".

The Omnitrix – The alien wristwatch that the whole franchise is based around, with it, the user can transform into the absolute prime form (the healthiest, strongest and best versions) of many different alien species that is known of in the Codon Stream, and can even add more alien DNA that wasn't present in it by "scanning" them if they touch the watch. It was created by the smartest alien in 3 (arguably 5) galaxies.

The Codon Stream – The Omnitrix is like a wireless receiver, when the user selects Alien DNA to transform into, the Codon Stream on the Planet Primus sends it over to the Omnitrix. Without the Codon Stream, the Omnitrix is useless…

The Null Void – An alternate dimension outside the normal flow of time where the worst criminals in the universe are banished, and was used by the Plumbers to seal these very dangerous aliens away. It also houses a huge prison for these inmates, though if they escape those institutions, they cannot escape the vastness of the void. Its landscape is always changing, so it's actually impossible to know where you're going.

Plumbers – Essentially the "Space Police" of Ben 10, they operate throughout the galaxies under their jurisdiction, finding offenders and preventing crime. Most of the very dangerous offenders are sent to the Null Void through a Null Void Projector, a gun that can shoot open a portal into it.

Touhou

Gensokyo – The land where the entire Touhou Project series takes place. "Its culture vaguely resembles that of feudal Japan, with a lot of folktale elements added on."

Reimu Hakurei – "The main protagonist along with the deuteragonist, Marisa Kirisame. As the shrine maiden of the Hakurei Shrine, she manages the Hakurei Border of Gensokyo and exterminates troublesome youkai."

Marisa Kirisame – "An ordinary human magician who specializes in light and heat magic and currently resides in the Forest of Magic. Her motto is: "It ain't magic if it ain't flashy. Danmaku's (Bullet Curtain/Curtain Fire) all about firepower," and she has a compulsive mania for collecting things."

The Hakurei Shrine – "A small, run-down Shinto shrine located on the Great Hakurei Barrier on the east side of Gensokyo. It's a key location and the home of Reimu Hakurei, and has become a popular hangout for all sorts of youkai who Reimu and company defeat in the course of the Touhou games. However, because of this, very few (normal) humans come to the shrine, and even fewer (if any at all) leave donations."

Information taken from the Touhou Wiki.

Again, I hope you all enjoyed this chapter, and I will see you all soon!