Author's note: I'm broke and therefore own nothing of Kingdom Hearts or Naruto directly. They belong to their specific owners. This is the only time I will say this.
In the dark of night, a young boy leapt from tree to tree. Headed to an old cabin, the boy hefted the large scroll on his back and accelerated to inhuman speeds. After a few moments of travel, he comes to a stop and lands on the ground. He steps into the light, his garishly orange jumpsuit and striking blonde hair revealed. Grumbling slightly, the blonde boy known as Naruto Uzumaki let the Forbidden Scroll fall to the ground. Unrolling the scroll, he began to read, slowly and carefully. "Shadow Clones? Well, maybe it can get me out of the paperwork when I become Hokage… wait what's this?" he muttered confusedly. As he read on, Naruto stumbled across a section blatantly labeled 'Unknown Items'. "I wonder if one of these is a super awesome weapon!" he exclaimed. Smiling, he channeled chakra into the first seal. A poof of smoke revealed… "just a key chain?" Naruto muttered, dissatisfied. The keychain was a simple leaf symbol attached to a short 6-inch chain, all sterling silver. A shadow flickered into view at the edge of the clearing.
"NARUTO!" yelled an irritated man. His brown hair slightly disheveled from its usual pineapple shape, Iruka Umino was not a happy man. "WHY ON EARTH DID" he began to yell, only to be cut off by Naruto tackling him to the ground. A shadowy blob flew by through the space where the teacher's head once was. The blob landed a few feet away, revealing a black vaguely humanoid figure. It was very short, maybe 2 feet tall, and rounded almost like a child's drawing of an insect, complete with 2 curly antennae. It's glowing yellow eyes stared into Naruto's sapphire blue ones. Then the unexplainable happened. Naruto felt his chakra flow into the keychain and with a flash of light, something manifested. In his hand was a weapon, seemingly a demon windmill shuriken with a single blade. From that blade, a beam of pure blue chakra as wide as the blade extended about 1 and a half feet, ending attached to one of the 4th Hokage's legendary tri-pronged kunai. This strange new weapon generally resembled a key. Acting on instinct, Naruto swung his new key at the strange creature, dispersing it into a black mist. Before either the teacher or the student could question these events, a scream rang through the woods. Suddenly alert, the two ran to the source to discover a small horde of these… shadows. In the center of the mob was a screaming man with pale hair mostly hidden under a bandana. His scream cut off as he was consumed in black mist, his heart stolen away. "MIZUKI!" cried Iruka, scared for his co-worker. Iruka turned to Naruto. "Naruto, use that key. We have no choice but to fight. I'll back you up." The boy grinned.
"I've got an idea." Channeling more chakra into the key, Naruto closed his eyes as he felt the raw power flow back out into him, surrounding him. Iruka gasped as a shadow of the Fourth Hokage appeared behind the boy, seemingly made of pure chakra. Naruto's eyes snapped open, words called to his lips by the power raging through his system. "FLASH RAID!" He called, throwing the Key in a sweeping arc. Disappearing in a flash of yellow light, the boy caught the key on the other side of the horde and threw it again. Flash after flash, Naruto appeared all around the clearing, until nothing remained. Poor Iruka was so shocked that he could do nothing as Naruto collapsed, his chakra exhausted.
Naruto blinked awake, floating in water. Standing up, he found himself in a sewer with no clue how he got there. "Come to me." Naruto shivered as the deep demonic voice passed through the hall. With nothing better to do, he followed the voice until he ended up in a massive room. Half the room was shrouded in darkness beyond massive gold bars. On the bars was a massive tag with the word seal written on it in Kanji. "Good, you came." The deep voice rumbled from behind the bars. "We have much to discuss. Most of it has to do with your little Keyblade. To start, your world is breaking."
Naruto stared into the shadows for a few seconds before screaming. "WHAAAAAAAAAAT?"