Warning for just this chapter: Spoiler for those who do not know the Sage of the Six Paths and the whole Yu Yu Hakusho series (just a little). Also for Yusuke's colorful language in future chapters.

Edit (7/2015): Hey guys! I wrote Little Things a few years ago, back when I didn't really understand the characters' personality or how to shape a plot in a story. Needless to say, that means that Little Things really isn't the best example a YYHxNaruto story could be. As a result, some years later after Little Things ended, I decided to rewrite a shorter, more in-character version of Little Things, known as 'Little Things (that might have happened)'. I strongly suggest that you read that one instead of this one, but regardless of which fic you'll choose to read, please enjoy and thank you for reading.

Yusuke growled savagely, his spirit and demon power bubbled at the surface, barely contained as he felt more and more frustrated with every second. He cursed. He should have brought some kind of equipment with him or at least someone better at sensing energy. It's going to be years before he finds Kurama.

Sure, the two had grown closer over the years, bonding over the pain of knowing that their human friends and family will always leave before them, but it doesn't mean he'll be able to sense the fox half the globe away. Especially since it has been centuries since he last seen Kurama.

Yusuke clenched his hand into a fist, sharpen nails digging at his palm as they unconsciously grew at his agitated state. He blinked absentmindedly at the dull pain and raised his hand to eye level, where he could see long, pointy nails. Sighing, he ran his hand through his recently shorten hair. He will have to train his energy control again. After being in the Demon World for centuries, he had gotten used to not needing to restrain his energy and as a result, became lax in his control.

Now that the barriers between the two worlds have finally came down, he would have to polish it back up again if he doesn't want the humans to freak. He rolled his eyes. They always do, at anything that's not "normal." Hopefully, they'll be more like Kuwabara or Grandma instead, seeing they're their descendants and everything.

It all happened a few centuries ago, you see, when the Human World ended. A few allied countries were testing atomic bombs, planning to use it to rule the world or something. After a successful test run, human nature shone through and decided they really didn't really want to share. In secret, they remade a few of those destructive bombs and when one of the countries was at odds with another and declared war on them, an atomic bomb was used. It triggered other countries in using their nuclear bombs and set off a chain reaction. By then, around three-fifth of the world was destroyed. Of course, luck wasn't on mankind's side and the radioactive particles were carried on wind currents and across to other countries, thus affecting the other two-fifth.

Radioactive elements affect all organisms, and so Koenma immediately put up the barrier between the worlds. Humans weren't the only casualties though. Ever since the Demon World began to slowly integrate with the Human World, more demons began going to the Human World in peace. A few thousand demons were left in Human World when the barrier was pulled up, along with Kurama, Yukina and all the humans that Yusuke came to know.

When Yusuke learned of it, he demanded Koenma to let him go back, but the prince refused to risk anymore. At the time, had it been a decade ago, Yusuke would have been adamant about it, threatening Koenma bodily harm should he not listen. But it wasn't a decade ago and Yusuke was older and far more wiser. He understood how hard it was on Koenma, and knew the prince wanted him to save their friends too.

It wasn't like any other opponents Yusuke had fought to save the world, though. It was radiation. How does one go about saving the world if the thing threatening it isn't even tangible? Allowing Yusuke to go is just simply sending him to his death and risking a few more thousands of demons. It was a lost war.

So in the end, Yusuke stormed out of Koenma's office and forced some poor fool who was at the wrong place at the wrong time to open the portal and gone back to the Demon World. That day, the Human World was destroyed, followed by a fifth of the Demon World's forest courtesy of Yusuke's punch and Hiei's sword. And that was only because they were fighting each other and not the forest, because if they were, the forest would have lost to the two enraged demons. Badly.

After the fight, Yusuke and Hiei was injured to the point of needing months to fully recover. By then, their anger bled out and all that was left was remorse (although Hiei would never admit it). Survivors learned to fear the two, since that day was nearly marked "The Day Human and Demon World Ended."

It was all due to the incident that allowed Hiei and Yusuke to grow closer together as well. Both had lost people important to them, allowing them to understand each other on another level. The two could also be found often sparing with one another, although it was nowhere as intense as their last battle. When Koenma informed them the barrier was down, Hiei immediately went ahead to scout and came back with information on the new Human World. Or perhaps it should be changed to Ninja World, much to Yusuke's amusement.

After much of the world was destroyed, Genkai went around saving the survivors. Not much of her style, but she hardly wanted the human race to end and without a leader to guide them in adapting, that would have undoubtedly happen. She first went to the areas that were affected the least by the radiation, using her powers to protect lives, but only that since there were others that needed her. The action, needless to say, caused mutation, but at least it wouldn't be life threatening.

Besides, survival of the fittest, meaning those who adapt would survive. With any luck, their mutation would bring about difference that will help them do exactly that. Genkai taught them how to survive and use their spirit energy (which came to be known as chakra), but the rest is up to them. Genkai didn't get out unscathed though. She, too, was affected by the little radiation around her, but not in a bad way. It was almost as if the world was trying to help her. You see, her mutation caused her spirit energy go back to the time when she was in her prime. As if the world understood that she was trying to help mankind and gave her the power to.

(Although the System aka the balance between the Spirit World, Demon World, and Human World is the true mastermind. Or course, the System failed to mention the fact that Genkai coincidently gained her boost of power the moment after Yusuke was knocked out of cold from his fight with Hiei and promptly caught a bad fever. The fever was nothing serious, but it prevented Yusuke from using his powers for fear of feeling dizzy and sick. If Yusuke's energy-particularly spiritual energy-was suspiciously low during the time Genkai was in her prime, it was only a coincident. It was not because the Law of Conservation of Energy, which states energy cannot be created nor destroyed, thus leaving the System pouting. The System especially did not cheat and lend Genkai Yusuke's spiritual energy by creating a situation where Yusuke wouldn't use it.

And of course, there was no way the System destroyed a little too much of Demon World's forest on purpose to make Hiei and Yusuke seem stronger (and make everything a little more balance) and therefore, demons won't dare strike them while they rest...and Yusuke without half of his spirit energy, but they don't know that. Nope, not at all.)

It wouldn't be too surprising if that was the case though. The three worlds are what balance each other out, allowing them to have equilibrium. With the Human World almost destroyed, the equilibrium would have been lost, which would eventually cause the system of the three worlds to collapse, and the other two worlds to self-destruct. If mankind survives, however, all that wouldn't happen. And that's where Genkai comes in, along with other high spiritual aware beings such as Kuwabara.

With their help, enough of the mankind survived, even if they did change. Of course, their saving the world adventure didn't end there. Unfortunately, while many lives were saved, many were also lost. Those who died had their souls floating around since Spirit World can't send people in to collect them. With enough time, the radiation would even decay souls. Around the time a semblance of peace finally reign, at last, the radioactive material was given enough time to decay souls. (It depends on the amount of radiation there is and how weak/strong the soul is)

When souls decay, they cannot go back to the cycle of reincarnation (they'll simply dissolve so they no longer have a 'mind,' so to speak, but their energy will remain in Human World), which caused a huge panic among those who witnessed the disintegration of the other souls. The panic continued to spread, until they fell back to instinct, which was to band together so they're one big soul. Since weak souls get decayed first, that was practically the only solution. However, after they combined to one whole being and the danger of being disintegrated passed, the souls began fighting over who controls the being. (Think of FMA and their philosopher stone)

While the souls within the being fight it out, the common factor of all the souls pilot: Animal instinct (Like Bleach's Vizards when they go into their inner world and fight their hollow). The great being (who, by the way, has Ten-Tails) followed its own sense of self-preservation and headed towards areas with little radiation. Alas, that was also where little villages were built by the surviving humans and eventually, the two crossed path.

And here is where historians of the Age of Shinobi were wrong. The Sage of the Six Paths was actually two people. One was Genkai and the other was Kuwabara. Genkai used her power and stopped the Ten-Tailed from advancing forward when it attacked. She never sealed it within herself and instead, told Kuwabara to cut it up into pieces with his spirit sword. He ended up cutting them into nine pieces, and Genkai quickly used the remaining of her power to seal the rest. After that, she had virtually no spirit energy left and as a result, passed away.

Kuwabara was the one with the noble intention of establishing peace throughout the world, since it was the nuclear war that torn the worlds apart, along with the people living in them. Yukina was one of the billion of victims of war. The Ten-Tailed's potent and poisonous chakra affected her during its rampage and although Genkai stopped the poison from spreading any further before her own death, Yukina still went blind and her eyes turned featureless white.

However, a few weeks after that, she realized that although she cannot see people the way she did before, she could see a lot of colors in human shapes. Her 'vision' also extends on so she could see an area of at least 50 meters from all angles. It also gave her an incredible clarity of perception, and as a result, could memorize things more easily. In addition, with her eyes, she could tell when someone was sick since the flow of the color of their body would look different, and used it to heal.

After that, the people lived a relatively peaceful life and the tiny villages began to grow. Children were born, some with weird abilities due to mutation, some not. Yukina and Kuwarbara's first son fit in the first category. He apparently inherited Yukina's eyes, even though his are fine. The family was so happy for the gift that they named this inheritance Kekkei Genkai, naming it after the person that saved Yukina and many others, as well as teaching them how to use their spiritual energy/charkra. The second son, on the other hand, took after the father. Although he had no special ability besides an incredible amount of chakra reserve, he had Kuwabara's noble intent, along with his willpower or Will of Fire. It was the older son who decided to follow his father's step and use a sword though.

Yukina sadly died before her children's tenth birthday, having to be affected by the radiation more than she realized. Kuwabara was devastated, but was unable to remain in that state long since he still have two children to care for. He took care of his children as well as he could while still managing to look over the villages' affair. He also went to check on the separated parts of the Ten-Tailed to make sure they're still tightly sealed.

During one of his visits, Kuwabara realized Kurama was a part of the sealed beast when the shapeless lump of souls morphed into a fox with nine tails, since it was he who won the battle among the souls within that lump. The other eight remains of the Ten-Tails will eventually follow when a victor emerge and will too morph into the shape of that victor, but not all will reach a resolution during Kuwabara's lifetime.

On his deathbed, Kuwabara chose his younger son as his successor to look over the villages. Little did he know, the older son will soon attack the younger, beginning a war between them shortly after his death. The Tablet was created afterthis happened, during the second generation of this newly created world by someone who wanted their future generations to know what happened. They didn't know the true story, however, and automatically assumed Kuwabara was the one that dealt with all the problems. (Gender discrimination, unfortunately, lived on.)

If the idea of "radiation ended human world" sounds familiar, it should. I got the idea from TheBeingOfEverything's The World Ends With Humans. It's awesome.

Note: I don't know is where this is going, or even if it is going anywhere. The only thing I have planned out is the detailed background of how YYH and Naruto merge, which is presented already. So please don't ask any question about when I'm going to update, if this or this is going to happen, is he or she is going to appear and stuff like that. I don't know. I'm a rather clueless author.

However, feel free to make any suggestions. It doesn't mean I'll put every single thing offered to me in the story, but I will if it triggers my fingers to type. Of course, I'll give credit to where it is due. But do keep in mind that this will be a series of one-shots that are loosely connected.

Lastly, I apologize for any future ooc, since I'm not updated with Naruto and it has been a while since I last watched YYH. I got some of these information from their respective wikia and the rest from my imagination.