EDIT: Since Dragon Ball Z is a kid's show, I decided to keep this general, lol.

Buu is a universal constant of destruction in the Dragonball Universe. No matter who, or what he encounters, he always destroys it. Even the Kai's who are meant to be God-Lords of reality in their own right. Several have met their end to him. Others were absorbed. He was a very proud being concerning all that he destroyed. He took orders from no one. His scream alone was so powerful that it could shatter reality and puncture holes in time-space. Some of which even led to other dimensions. One time, when he was rampaging as Kid buu, he heard another, fiercer roar than his own pierce the realm. Rocks rising in his anger disintegrated. Waters disintegrated; not evaporated, but disintegrated at the molecular level! Reality itself seemed to turn sideways. Then there was a deep, menacing voice.

"SHUT UP, YOU DEMON BRAT!" A large, orange fox said as it appeared on the other side of a spacial rift. Kid Buu immediately flipped back up and looked at the orange figure, petulantly slamming his fist against his chest to declare dominance. The Kyuubi opened his massive mouth in a rage, tearing through the rift into the realm of Dragon ball, ready to destroy the pink wad of chewing gum. The very instant that he crossed into the realm, his bestial aura filled the realm with his chakra. Buu fell silent as he felt the unfamiliar power stretch through his body! He was speechless.
The majin had existed since the creation of the universe! How could he not have known of such a power? Still. It was nothing compared to his. At least, that's what he thought until the equivalent of a spit ball shot out of the fox's mouth. It hit him immediately, going off like a megaton bomb and making the rock features shatter.
The majin didn't move an inch, but now he wanted to fight. It was all he was good at; all he did and liked.
So he shot his own energy back at twice the size. The fox spat it away as well, knocking it into a nearby ocean.

"You must have a death wish!" Kurama said, not caring that he didn't know where he was. The rifts closed as he approached the kid, and he stood on his hind legs to punch the kid. Buu extended his arm before Kurama could put his weight into it, stopping his fist before it launched forward and knocking him back!
"WHY YO-" the fox yelled, before an even larger energy ball hit him in the stomach, knocking him into the ocean. The blast was as large as a city, and the 9-tails lie at the center of it.
Buu laughed at defeating the fox in such an abrupt way... until the fox jumped out unscathed and launched a blast of the same proportion at his feet. The careless majin didn't bother dodging, and got a mouthful of energy as the blast echoed through the land. Kurama didn't stop at one either. He was PISSED!
"TAILED BEAST BARRAGE!"
/BOOM/ /BOOM/ /BOOOM/ /BOOOM /BOOM/ Several large explosions went off where the majin was standing, sending pillars of smoke into the air for several minutes. Kurama shook himself free of the water and gave a contemptuous "hmmph".
"No one goes Madara on me and lives to- GAAAH!!!!" a pinch, enlarged hand shot out of the ground and grabbed the fox by the throat; launching him into the air by several hundred meters before slamming him HARD into the ground. Then again, and again, and again. 7 times in total before his limp body was brought back to the pink monstrosity, who was admiring his handiwork. Seeing that the fox wasn't responsive, he launched a final energy blast, completely destroying the fox.
Kid buu cheered and laugh at another win, like a perverted wrestling champion to his crowd... that is until a bright light appeared from behind him. He immediately turned around to see the fox's arm re materializing, cutting his gooey body apart while the rest of the fox re-materialized. Kurama was immortal. Killing him just made him resurrect.

The majin didn't realize this, and left his lower half on the ground to back away a little. 9 orange blurs lashed out and grabbed him, before slamming him into the ground and pummeling him. The 9-tails tails had enough power to destroy MOUNTAINS, and he was using all of them on the stupid pink wad. When it was all done, there was pink goo everywhere. In the jagged crater, in the air, even on the fox's tails.
The 9-tails released a savage, screen breaking roar of anger as he vented out his rage! His body oozed with Sage Energy, amplifying all of his abilities and strengths. "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAH!!! D*mn BRAT!" he finished, before looking at HIS handiwork. Nothing that remained of the blob was infuriating to the fox anymore. There small blobs of goo were practically dust bunnies before his rage.

The fox released a blinding white blast of wound, turning everything before him into fine dust in the vibrations!
With that, he turned away, having enough of the fiasco. He was just about to dig the spacial rift back open and leave when he noticed a pink mist gathering before him. In just a few seconds, it began to resemble the pink wad.
/WAAAAAAAM/ "STAY DEAD!" he yelled as he stood up and slammed his hind-paw down on the blob. The ground shook apart, forming massive faults, but it was no use. A pink laser spouted up and sent his paw flying into the air, and him along with it. It righted himself as Buu ascended back up in the laser, body reformed and ready for more entertainment.

He was having the time of his life, having met someone who could actually SURVIVE what killed many others. Kurama on the other hand was infuriated! He had never met such a resilient being who pissed him off!
"What is he?" Kurama asked himself as he began building his chakra again. Buu began charging a massive blast, even larger than the ones before. It even looked different, spinning furiously as it let out an ominous hum.

Kurama stood on his hind legs and drew his tails above his head, gathering a massive ball of chakra and condensing it in his mouth. The ground shook furiously at both of their attacks. The Kyuubi was too mad to let him finish charging his attack, and all the more, he was pissed that this kid wouldn't leave him alone. With almost no warning, he bulked up his muscles and activated one of his 8 gates; something he hadn't done in eons. His body erupted in steam as he leaned forward, hands on his knees as he aimed the attack. Buu launched his, sending the land into disrepair. Kurama launched his high density attack as well.
There was enough power in Buu's attack to destroy the planet 10 times, but Kurama's attack was so fast that it deflected it, sending it, and his attack rocketing into space on opposite sides of them. Kurama's massive blast destroyed several mountains before erupting into a massive fireball. Small bits of it hit and burned kid buu as shrapnel, making him yell and scratch in pain. Kid Buu's attack exploded at such a massive scale that it tilted the planet off its axis, making the entire surface receive 200 miles an hour winds before it righted itself! Kurama was sent several miles away, and blown into the upper troposphere, which filled with dark clouds and lightning.
"WHAT IS THIS?!" The kyuubi yelled madly as the entire planet shook apart around him. Just as he righted himself in the air, he saw the pink blob launching himself at high speed, in the form of a cannonball. It it the kyuubi smack in the face, then the gut as it turned. It began pummeling him in every direction, until finally, Kurama went limp and fell to the ground again. This time, when Kurama hit the ground Kid buu stopped himself mid-air, and launched an EVEN LARGER attack. It wan't as large as the planet busting madness ravaging the land, but it was definitely as large as a city block!
The 9-tails managed to get air back into his lungs, and stand up in time to see the attack come at him! He had bruises all over his body.
"Grrh!! Can't block that!" he said as he weaved hand signs. It headed toward him at terminal velocity, with Kid Buu still holding it like a madman. He was about to slam dunk on the kyuubi, until Kurama activated the Justu.
"TRANSFORMATION JUTSU!" /POOF/ the instant the kurama yelled, the ball disappeared in a cloud and turned to a cloud of air. Still, that air hit him at high speed, knocking him onto the ground. Kid buu narrowly missed him, diving right between his legs and landing a few meters away from the gasping fox. He had no idea what just happened!
The kyuubi turned around as Buu launched even more energy balls; smaller, but much faster.
"Transformation jutsu!" /Poof/ The air in front of him turned into cloud trampoline, bouncing them all back at Buu, who was surprised. They all went off as he staggered, caught off guard by his own attacks!
/BOOM/ /BOOM/ /BOOM/ /BOOM/ /BOOM/ /BOOM/ /BOOM/
the loud blast cleared the sky of all clouds, and the debris pelted Kurama, making him fall back again. He was beginning to tire!
Once again, Kid buu reformed in the cloud.
The 9-tails forced himself to stand, knowing that this wasn't going to end until he was permanently sealed or destroyed.
"Nngh, Blasted brat! I'LL SHOW YOU POWER!" The 9-tails said as he summoned two shadow clones. They both knelt down, tired but staying still as they gathered more nature energy. Buu was taking quiet a while to reform, as his own blast had sent him to bits and pieces over several miles.

As his body fully formed, Kurama's clones sent him more nature energy, enabling him to stand and shake off the brutal assault!
Buu frowned at him, knowing what was coming. He was quick to notice that kurama detected every movement he made when he had that energy, and decided to interrupt with a long, extended punch. Kurama blocked it and pulled his fist toward him, into the range of his two clones. Buu grunted nervously as the clones fist lit with fire and started assaulting him.
"KITSUNE FIST!" they yelled, trying to hit the small, fast, and very resilient blob. He stopped holding back at this point, outmaneuvering their seemingly slow attacks. While the clones were busy fighting him, the main kurama charging up another jutsu. Brute force clearly didn't work, and he was on his last ropes.
Buu grew bored of the clones after a few seconds and blasted them away. Two more poofed into existence and began attacking him. Then several dozen. They all fought him with the little bit of strength they had left, while the real one began running away to hide and charge his jutsu. Anything that could destroy a planet was a force to be reckoned with. He was stopped in his tracks by what remained of buu's lower half when he had first resurrected and sliced through him!
The 9-tails kicked furiously at the expanding slime, but it was no use. It creeped up his body and wrestled itself to the ankles of his long digitigrade paws. Kid buu was rather small in comparison. Normal absorption would take time... but he only did this to throw kurama off his game.
The clones fighting him had managed to seal him off in a red barrier, to which buu blasted and punched with great force. Insomuch that the clones were still hit by the force through the barrier as he slowly pummeled cracks into it. When the original was being attacked, several of them lost focus for a brief moment, which is about 7 seconds in dragon ball z time. Kid buu released a massive blast in the barrier, shattering it and destroying them before rocketing toward the real kurama, yelling with a smile only homicidal a mother could love.

Kurama cursed as his clones dissipated; hands still trying to perform his jutsu. Kid Buu saw that, and wasted no time ramming him into the ground. The crater was so large that nothing else could be seen, as the hyper-sonic monster collided with the kitsune. The smoke quickly cleared, revealing Kurama unconscious and severely bruised in a smoking crater. Kid Buu restrained himself from celebrating, knowing that he would just resurrect if he finished him off again. He had a different plan in mind now. With a stretching forth of his arm and lifting of one finger, his lower half that attached to Kurama's paw exploded forth and rapidly began spreading over the rest of him. It was up to his waste when the 9-tails hands moved from his sides and tried to form the final hand sign. Buu immediately launched the goo creeping up his waste between his hands, baking it impossible to form the seal. Kurama cursed as the slime sapped his strength and made his arms fall down in front of him. Still, he weakly tried to grab and stop it. Buu teleported onto his chest and stomped, HARD. The Kyuubi hissed as his hands lost their grip and dropped to the ground. His head hit the ground as he lay flat in the crater. He was going unconscious as Buu uttered actual words. "Join Buu. You Kill, I destroy... Pet." he said to the fox as he tried to prop himself up again; buu just in view on his chest. "I ain't... your D*m... pet!" he weakly mouthed as the slime wrapped up to his chest; Buu becoming a part of it and gushing over him.
The fox was too weak to resist now, and most of his body was covered when Bu started forming over it. His paws turned a pink shade as they were assimilated, as well as his tails.
Pants resembling Kid Buu's pants formed around his lower half, with braces and a collar on his arms and neck.
A strange euphoria rushed over the Kyuubi's mind as the goo covered him. Happiness. He felt alive... free to destroy whatever he wanted and eat whatever he wanted... aside from Buu. He was his... master. Master who saved him from the eternal nightmare of the ninja world, where he was hunted day and night by shinobi who wanted his power. They would never find him here... nor could they capture him. In his captured heart, Kurama was admitting that Buu was his master now, and enjoying it. A new, frightening level of power and kid buu's voice filled his body; his strength returning as he sat up.
"You belong to him. He owns you. You serve him, and love serving him" the voice said, as Buu reformed from the slime and sat on the head of his pet.
"Come. We eat now!" Buu said to the hypnotized fox.
"Yes... master" the fox said with a smile as buu jumped off and flew off to ravage the rest of the planet. Kurama did likewise, following his lead, and not trying to resist his hypnotic magic.

This was actually one of the LONGEST pieces it ever took me to make, and then I had to add a story to it for it to make sense :O