Author's note: This is a story written not by me, Animercom, but my brother. Though it is written by him, I have his permission to post his story under my account because he does not have an account of his own. Henceforth, all interactions with the readers involving this story, including PM's, will be written by him and not me (Animercom). This story takes place after the events in the Yu Yu Hakusho anime (manga works too but the differences aren't mentioned) and after the middle of volume four of assassination classroom, right after Koro-Sensei went on his trip to the movies with Karma. In other words, just before he meets the second transfer student.

Yu Yu Classroom: The Spirit Detective of Assassination!

Episode One: Homecoming

A sigh escaping his lips, Yusuke squinted up at the sun. It was time to come to terms with a fact he'd been avoiding for several months now.

His life sucked.

Really sucked.

Sure, he was alive despite his fierce battle with Yomi, one of the three former Kings of the Demon Realm, but Yusuke was bored out of his mind without anyone to fight in the Human Realm. He couldn't even bring himself to brawl with his fellow students as it just didn't give him the feeling of excitement that his other battles with demons always had. Even the "favors" Koenma had come to Yusuke with were dull and far below his level. Recently the most entertaining thing in his life was when Yusuke discovered he could channel his Yoki into a spirit-like body while sleeping. Feeling a bit mischievous, he had sent it up to the Spirit Realm and terrorized its citizens by pretending his demonkin blood had driven him mad with hunger for their souls. Yusuke couldn't help but smirk as he recalled Koenma cowering behind his desk, begging for his life. But if he pulled that stunt again, he'd probably be banished to the Demon Realm. After all, Koenma could never take a joke.

"Hey, it's not like you to look like you're thinking deeply about stuff," his friend Kuwabara remarked. He was walking next to Yusuke on the way to school, just like they usually did. For once, he was so quiet Yusuke forgot he was even there.

"I don't need to hear that from you," Yusuke retorted without turning to his friend. Kuwabara merely grunted in response. Knowing he wouldn't get away with such a simple answer, Yusuke explained, "It's just too quiet around here."

"Eh? What do you mean?" Kuwabara's voice raised several octaves higher than any normal man's as he added, "The locusts are making such a racket that they wake me up in the morning more than my own alarm clock!"

Yusuke yelled, "Not that, dumbass!" In a huff, Yusuke turned to face Kuwabara. "Despite the barrier blocking the Demon and Human Realms being lifted, none of the demons are causing trouble. No fights have broken out, no worthy opponents have risen up…" Yusuke clenched his fists. "I still haven't found my satisfaction…"

Yusuke had been talking about "satisfaction" for a long time now. It started when his climatic fight with Sensui was interrupted by his ancestor, a demon. Claiming that he couldn't watch as Yusuke struggled against Sensui, the demon had possessed Yusuke and finished the fight in his stead. Yusuke swore he'd find his ancestor and pound him until he got back the satisfaction he was so wrongly denied. But instead he got thrown into the Demon Realm's politics and history. It was a world his simple-mindedness couldn't quite follow. Eventually, Yusuke decided he'd put a rest to the Demon Realm's problems by hosting a tournament, but he couldn't even win the tournament that he himself came up with! At first, he'd been satisfied to just say "I'll win next time!" but Yusuke wasn't a patient man. He would have to wait two and a half years before another tournament would be held in the Demon Realm.

Kuwabara frowned. "Your satisfaction, eh…?"

Yusuke walked on without checking to see if Kuwabara was following him. "Don't tell Keiko this, but I've been thinking about training in the Demon Realm for a while to blow off some steam."

Kuwabara shook his head. Only Yusuke would think of training in the Demon Realm as a means of relieving stress. "And just when did you think you were skipping out on us this time?"

"Who knows? Could be an hour, could be a month."

"I see…"

Yusuke glanced over at Kuwabara. He seemed a little sad, but unsurprised by Yusuke's announcement. It made sense. Kuwabara hadn't seen much of Yusuke since his misadventures in the Demon Realm. Come to think of it, neither had Keiko. Or that old hag. Or his mom, or…

Maybe he shouldn't abandon them all by going to the Demon Realm again. But if things didn't start getting interesting soon, Yusuke thought he'd snap…

"Everyone, terrible news!"

Startled, Yusuke looked up at the source of the voice. A figure was flying towards them with an incredible speed as though adrenaline were pushing it forward. Kuwabara gasped. "T-That's…"

"What are you doing here, Botan?"

Botan landed in front of Yusuke and Kuwabara. She was dressed in a school girl's outfit, so if any passerby happened to glance at the trio, they'd look like an ordinary group of friends on their way to school. But Yusuke knew better. Botan was the Grim-Reaper. In sharp contrast to her profession, she was normally a rather lively and happy person (which to Yusuke meant "loud and annoying"), so for her to show up so flustered couldn't have meant anything good.

Botan put her hands on her knees and tried in vain to catch her breath. "Koenma-sama… Koenma-sama has a new assignment for you."

Yusuke regarded her calmly as Kuwabara said with excitement, "Hey, Urameshi! This could be what you were waiting for, yeah? A new demon to fight, and an entire Realm to save, perhaps? Maybe all three Realms? We could get the guys together just like old times and-"

Yusuke ignored Kuwabara and walked passed Botan. He said nonchalantly, "Koenma's the one who fired ME, remember? I don't have to take on any assignments from him anymore, and that includes favors." When the Spirit Realm had discovered that Yusuke was related to a Demon, they stripped him of his title Spirit Detective, a title Yusuke had become somewhat attached to. It miffed him more than he'd like to admit when it was revoked so suddenly because of circumstances that were beyond his control.

Botan cried, "But all of the favors we asked you for were critical for the wellbeing of the Realms!"

Yusuke yelled, "Bullshit! The last one had me searching for the Demon who had ran away with Koenma's Sacred Boxer-shorts. Kuwabara, we're leaving. If she hears we were late again, Keiko will eat us alive."

Botan stepped in front of Yusuke with a seriousness that surprised him. She glared up at him as she said ominously, "What if I told you this new assignment involved defeating a foe unlike anything you'd ever seen before? And that if you failed… the entire Human Realm will be put at risk."

Yusuke stared down at Botan. "What do you mean?"

Botan let out a sigh of relief. "It's a long story. Do you have time?"

Yusuke half turned to Kuwabara. "Hey, what time is it?"

Glancing down at his watch, Kuwabara stated, "7:12"

Yusuke threw his suitcase behind him, excitement starting to build. "Then we're skipping school today. We'd never make it in time anyway."

"H-Hey, I thought you said Keiko-"

Startling Botan, Yusuke called, "Hey, Botan! Let's hear this assignment." He added with a glint in his eyes and a smirk that only a demonkin could muster, "Things are starting to get interesting."

The trio had decided to hear the full scoop at a local fast food joint. Kuwabara sat next to Yusuke in their booth seats, and Botan sat across from them. Only Yusuke had the appetite to order anything, so he bought a shake.

Slurping on the shake loudly completely unaware or just plain ignoring the annoyed glances of other customers, Yusuke said, "So, tell us. What's so threatening that the entire Human Realm would be in danger?"

Botan leaned back in her seat. "Where to start… Have you seen the news reports about the destruction of the moon?"

Who hadn't seen them? Half of the moon was obliterated out of nowhere one day, causing mass panic all across the world. Yusuke heard a rumor from his friend Kurama that the Spirit Realm launched a huge manhunt for the perpetrator, but since they couldn't pick up any Yoki or Reiki from the moon's remains, they gave up quickly. Apparently, they said it was a human affair and that demons weren't involved at all, so it wasn't their business. As if humans had the power or the inclination to destroy their own, but Koenma was Koenma.

"What about it?" Yusuke asked. "That was well over two months ago."

Nervous, Kuwabara squirmed in his seat. "Don't tell me they found the guy who did it?"

Botan nodded. "Yes. A week ago, the Spirit Realm discovered the culprit, and he was nothing anyone could have possibly imagined." Yusuke and Kuwabara found themselves leaning forward subconsciously. Anyone who could destroy half the moon in the bat of an eye and then hide themselves from the world for months had to be a hell of a fighter. Botan continued, "…It was…"

"It was…?"

"…An octopus."

Kuwabara burst out laughing and Yusuke stared at Botan, dumbfounded. "Ehhh?! Is this some kind of a joke? It sounds like the sort of crap from a shoddy Shonen manga!"

Botan threw up her hands. "Believe me, I was shocked too. But it's true. Koenma-sama hadn't been satisfied with Yama the Great's recall on the manhunt, so he's been sending out search teams every once in a while to look into anything that might be a clue. When he heard stories of a tentacle monster attacking a famous American baseball player and watching movies in movie theaters, he sent a team and-"

Kuwabara interrupted her. "Whoa, back up. You're not joking? It was seriously an octopus? Like, those pink things with tentacles that can shock your brains out?"

Yusuke glanced over at Kuwabara dubiously. "Aren't those jelly-fish…?"

Ignoring Kuwabara's inability to follow intelligent conversations, Botan continued, "…So after investigating the reports, he found out they all point to the same place: Kunugigaoka Junior High, Class 3-E."

Yusuke laughed. "Let me get this straight. You're telling me some students fed their class pet some mutated fish food and now it's rampaging across the world and destroying the moon?!" Yusuke doubled-over. "That's about as believable as the time you said you'd give me tickets to that sports championship!"

Botan puffed out her cheeks. "This is a grave matter! If you won't take it seriously, the Spirit Realm will find someone else to handle it!"

Wiping tears from his eyes, Yusuke apologized. "Sorry, sorry…" At least she tells entertaining stories, even if it's probably just a prank for threatening to eat the Spirit Realm's inhabitants…Yusuke said aloud, "Go on."

Her tone noticeably more irritated, Botan huffed, "…Their teacher is a demon with incredible powers, including an ability to move at mach-20. He plans to destroy the world within ten months, exactly a year after he destroyed the moon, if no one can kill him before hand. In order to raise the world's chances of killing him, he decided to teach Kunugigaoka Junior High's Class 3-E students to be assassins. But even after spending two months with him, not a single one of them has come close to seriously injuring, let alone killing, him."

Kuwabara stared at the sugar packets on the table like they housed a tentacle that was about to wrap itself around his neck and strangle him. "This is… a lot to take in."

Deciding to humor Botan by taking this seriously, Yusuke scratched his chin and asked, "And? What in the world is the government doing?"

Botan shook her head. "I wouldn't count on them if I were you. So far they'd sent in some assassins of their own, but they're powerless."

Yusuke couldn't help but smirk. "So, in short, both the Spirit Realm and all of Japan can't do anything?" Yusuke rubbed the bottom of his nose. "All governments seem to crumble whenever anything truly important happens." Botan didn't respond to that.

Yusuke didn't intend to be rude by making that statement, he was simply ascertaining Botan's authenticity. If her entire story was a joke, then Botan wouldn't have hesitated to defend the capabilities of the Spirit Realm, thus contradicting her earlier statement about their incompetence and exposing her lie. But since she didn't even try to argue, that had to mean that, as insane as it sounds, an octopus demon with astounding powers really was trying to destroy the world.

Kuwabara either picked up on this too, or he grew tired of being doubtful of the story. "But what are we supposed to do about all this?" He crossed his arms and lowered his head, thinking. "I mean, sure, we've battled a lotta demons, but this guy sounds like he's on a whole other level."

Yusuke looked up at the ceiling of the joint. As he watched the fan overhead spin slowly and methodically, he said, "Yeah… if he were judged on the Spirit Realm's scale, he'd probably be ranked above S. Maybe he'd be ranked, like, T or something."

Kuwabara snorted. "Because T comes after S? You aren't very original."

Regardless of if the T ranking was a joke, Yusuke was serious about the perpetrator's power. If what Botan said was accurate, he had a speed of mach-20 and enough firepower to blow up the moon… And enough cunning to be able to hide his existence from even the Spirit Realm for so long. He may have been even more powerful than Yomi.

Defeating someone like that… would bring an unparalleled amount of satisfaction to Yusuke.

After rummaging through his pockets, Yusuke slammed down the money for his drink on the table. He stood up and grabbed his coat. "All right, I'll look into it."

Botan all but jumped out of her seat. "Thank you! Koenma-sama will be-"

Yusuke thrust his index finger in Botan's face. "But. Let's have ourselves a wager."

Botan's face clouded with uncertainty. "A… wager?"

Staring into Botan's expecting eyes, Yusuke slurped down the rest of his drink slowly and deliberately to build up hype. Once he was finished, he tossed it to Kuwabara, who stumbled to catch it. "Yeah. A wager." Yusuke stretched and cracked his bones as he explained, "To be honest, if I found out about this guy on my own, I'd have been all over the idea of hunting him down. But when someone orders me to do something, and presents it in a way that leaves me with no other choice, it leaves a bad taste in my mouth. A part of me doesn't want to do it."

Standing up and gesturing his hands like mad, Kuwabara yelled, "But this concerns all of the Human Realm! Don't tell me you're so petty you'd refuse and let the world get destroyed!"

Yusuke shook his head at Kuwabara. "No, I'll take on the assignment all right. I just want to make this even more fun!" Turning to Botan, Yusuke declared, "Bring Hiei and Kurama here. The three of us will try to kill this guy within a week. After all, if even trained assassins can't kill him in two months, then I don't want to waste any more time than a week trying to kill what may be an invincible opponent."

Her brow furrowing, Botan's asked, "And if that week passes and you fail…?"

Yusuke made a finger gun with his hand and pointed at Botan. "Then I'll determine that he's impossible to kill, and I'll tell him he can destroy the world immediately."

Silence.

As Yusuke walked out the door of the joint and onto the sidewalk, Botan and Kuwabara all but lost their minds. They ran after him and tried to yell over each other, both vying to be heard.

"ARE YOU INSANE?! Even for you, allowing some demon to destroy the world-!"

"And where do you get off making demands of the Spirit Realm?! You're lucky you're even allowed in the Human Realm after we learned about you being demonkin!"

"And who do you think you are, throwing your drink at me like I'm some sort of slave? Kuwabara the man is not-"

"Who even said that you're a good enough judge of a demon to determine if he's kill-able or not?"

Yusuke stopped walking. Without turning around, Yusuke asked, "Hey. This guy who blew up the moon… What's his name?"

Botan stopped yelling, surprised by the sudden question. "It's… Koro-Sensei."

"'Cannot Kill Teacher', huh?" Yusuke looked up at the clouds. "…We'll see."

Koro-Sensei turned to his class with a bright refreshing smile that contradicted his plans to destroy the world. "Greetings class! Today I have a special announcement for you." The students looked at each other and murmured their excitement, wonder, and, hopefully, love of their amazing teacher. After letting his announcement sink in, Koro-Sensei proclaimed, "Tomorrow, there will be three new students joining our class!"

Almost immediately, Karma shot up his hand and asked without being called, "Is this another one of those pain in the ass students like Ritsu was?" Some students looked down at their desk, embarrassed by his outburst, while others nodded in appreciation.

Taking advantage of his promise to not harm the students, the government created Ritsu to join Class E. She was a machine designed for the purpose of killing Koro-Sensei, but she had disrupted class with her never ending assassination attempts, annoying not only the students, but also even Koro-Sensei himself. He re-programmed her to have a heart, and now, she was just like the other students: an ordinary elementary schooler assassin. The students must have feared that Koro-Sensei wouldn't be able to simply rewire these new students to be cooperative like he had Ritsu.

"Now, now," Koro-Sensei said. "I think you and our new students have quite a lot in common with you, Karma."

Karma raised an eyebrow. "I doubt it."

Koro-Sensei gestured with his tentacles. "The new students are Urameshi Yusuke, Hiei, and Yoko Kurama. And what makes them special…" Koro-Sensei leaned forward and smiled an evil smile as green stripes enveloped his yellow face. "…is that all three are assassins sent to kill me by the head of the afterlife himself."

The class exploded into an uproar, and Koro-Sensei was enjoying every bit of it. He loved it when his students got excited.

Nagisa stuttered, "B-But what kind of people would assassins like that be? Are they even human?"

Koro-Sensei tilted his head, like the answer should have been obvious. "No. Only two of them are demons." That only put the class in even more of an uproar.

Tapping the blackboard with a piece of chalk, Koro-Sensei somehow managed to silence the classroom. "Now these three are all veteran fighters who have been in more fights than I have avoided assassination attempts." His eyes tilting into a slant, he added, "I expect them to be greeted as such."

Nagisa gasped. "You mean… You want us to…"

His face lighting up with a red circle, Koro-Sensei said, "Correct… let us show our new classmates the marvel that is our assassination classroom. Nurururuufufufu…"

Hey all, I first came up with this crossover after hearing from my sister that FF only had one crossover between Assassination Classroom and Yu Yu Hakusho. This was simply unacceptable, so I decided to write one of my own. Writing fanfiction is not something I plan on doing again, but overall it was a fun experience. The story is complete, and I'll post chapters once a week on Wednesdays. The story is ten chapters long, most chapters being a little longer than this one.

This isn't a story that focuses on a happy romp between the cast of the two series. I wrote it as though it were an Assassination Classroom arc where three newbie transfer students each took their stabs at trying to assassinate Koro-Sensei, each with their different plans and ideas. So unfortunately, you won't be seeing a lot of class time or side characters like Karasuma or Irina. Even Koenma only makes one appearance. If you came here to find a story focusing on character interaction besides just Yusuke, Kurama, Hiei, and Koro-Sensei, then you'll probably be disappointed and shouldn't waste your time. If you came here to see how Yusuke and Co. would try to kill Koro-Sensei if confronted with the challenge, then you've come to the right place.

I've tried to do my best to uphold what's canon in both series and keep the characters in character, but if you notice an inconsistency, even something as small as, "Hey, Karasuma doesn't call this guy _, he calls him _," then please point it out and I will correct it for future chapters (within reason). Thanks in advance.

Hope you stick with me to the end, and I hope you enjoy the rest of the story.