I own neither of these series, but they are fun.

Spoilers for Majin Tantei Nougami Neuro if you haven't gotten around to finishing that.

EDIT (6/4/17) – Polished this up a bit. Now I feel better.

Supernatural encounters for Ciel had become something of a normal occurrence. Beyond his own demonic butler, his staff consisted of a physically-enhanced gardener, a markswoman with ridiculous aim for a maid, an explosive cook, and a man who could converse with snakes. He had met Reapers, dealt with cultists, battled re-animated corpses… the list could (and likely would) go on. What the general populace remained ignorant of, he handled on a daily basis. All in all, not too many things caught him by surprise anymore.

This kind of supernatural culture shock did, however. He had never dealt with falling through a time-slip before. That those existed wasn't so surprising.

It was the world he was greeted with that shook him.

"Young Master, I do believe we have a problem."

Problem was an understatement.

Sebastian had returned from his quick scouting of the area to the empty room of a building they had quickly holed themselves up in. It was the safest place they could find in this suddenly bright and noisy world. His butler, capable of way too many things to name, had a perplexed and uncertain look on his face. Problem was definitely an understatement.

"I'm aware we have a problem, Sebastian," he bit out. "What I want to know is if you've found a way out of it."

"Regrettably, I have not," came the answer laced in frustration.

Ciel looked out the window in annoyance, eyeing the traffic in the street below. Carriages that moved without anything visible pulling them, lights without flames far too bright for any sort of lightbulb in production in his own time, buildings far too tall without the proper structural supports, characters he did not recognize dominating signs…

This world was not his own.

"I have discovered the time and place we have been thrown into, at least," his butler continued. "We are on the island nation of Japan in the early Twenty-First Century."

"Twe-twenty-first?!" he parroted as he turned quickly back to him in disbelief. Displaced by around one hundred and fifty years? Did England look like the world outside of this building, too?

"Yes," he affirmed, casting a curious gaze out the window. "Amazing what advancements humans have made in that time."

"I don't care about any amazing advancements, Sebastian," he barked. "I just want to get home!"

"Of course."

They stood there in silence for a few moments. Ciel tried to ignore what lay outside the window by looking around the room, but even this small place was foreign. The floors and walls and ceiling were entirely too plain even for an empty room. It was sterile. The only life it contained aside from them was reddish hue cast by the lights outside. Beside him, Sebastian gave the room a once-over, his expression thoughtful.

…And then he froze.

Ciel noticed the change quickly, because it was an action not typical to the demon who served him. Suddenly demonic eyes shot to the corner of the room, a knife smoothly flowing from his coat to the wall. There was a tiny screech and a twang in response.

"…What is it, Sebastian?"

He didn't answer, but he figured he wasn't entirely sure as they calmly moved to investigate what he had pinned to the wall.

Sliced cleanly through with the sharp implement was something of a cross between a human eye and a bug, though its insides were mechanical in appearance. His butler carefully pulled it from the wall to look it over more closely, but it suddenly vanished in haze of greenish aura.

"Evil Friday…?"

"…Sebastian?"

As he spoke, Sebastian's eyes went wide with a realization he was not privy to. Ciel did not like the look on his butler's face. He had never known Sebastian to be nervous, but he appeared just so.

"We must leave," he said too quickly, pocketing his knife and lifting Ciel off of the ground. His protests were met with a stern look. "We are in the territory of someone dangerous, Young Master. We should not risk such an encounter."

The young Earl froze.

"Someone dangerous?"

"It depends on who dares," answered an unfamiliar voice.

Sebastian went stiff as Ciel's uncovered eye darted to the other figure who had shown himself. Defying the very laws of nature, the colorful creature greeted them with a predatory smile as he stood on the ceiling as if it were no different from the floor. Wild green eyes focused on him before his demonic butler whirled around, and he was moved to the relative safety of the space behind him.

Ciel did not have the hearing his butler possessed, but he could swear an unfamiliar name roll quietly off of Sebastian's tongue.

"We have been displaced," the demon said guardedly to the other being in the room. "This was the first place we happened upon that we could collect our bearings safely."

"Ho?" he purred. "Is that the disturbance I felt?"

Ciel was struck by the arrogance that seeped through his words, and Sebastian's subdued ones in contrast. Obviously, this was another demon before them, and something about him had Sebastian on edge.

"Quite likely."

"And the reason you have been running around is…?"

He felt the unspoken words there. Running around in my territory. The Earl gripped Sebastian's coat tightly. His butler's nervousness was becoming contagious.

"I have been attempting to gather information concerning our unfamiliar location. I had not noticed your claim."

"No, of course you hadn't, insect," he laughed.

Ciel bristled at the insult. Insulting Sebastian was an insult to him as his contract holder. He couldn't hold back his retort.

"Who do you think you are?" he questioned, stepping forward against Sebastian's outstretched arm.

"Young Master, don't-"

"Your food seems to have some bark to him."

"Do you know who I am?" Ciel continued despite Sebastian's stern request to keep quiet. He was thoroughly annoyed.

"Do you know who I am?"

Ciel felt it suddenly consume the room. The presence of the being in front of him was all at once in every inch of the space around them, and it was such a commanding, terrifying presence that the young boy felt his breath seize in his throat. A fine tremble in his legs made them feel weak. Sebastian could be terrifying when he wanted to be, had far too little trouble ending lives, but this... this was on a wholly different level than he'd known.

This man was, without a doubt, a predator.

They took a collective step back. His butler's eyes were flashing dangerously, but his expression didn't seem to match. Sweat beaded his brow. This was clearly not a battle he felt comfortable participating in, and that lack of confidence frightened the earl.

"Neuro!"

The spell in the room was broken by another voice.

A young woman entered the room through the doorway, heedless of the tension, entirely normal in appearance except for the clothing that decorated her. She crossed her arms over her chest and leveled the colorful man with an exasperated look, blonde hair swishing as she tilted her head.

"Nani suru yo, koko ni?" she continued in a language Ciel didn't recognize, coming up to face the being as though nothing he might do phased her. His standing on the ceiling was not, apparently, unusual. "Watashi no obento woACK!"

He descended from the ceiling directly onto her back, crushing her beneath him deliberately.

"Kuchi wo hasandeiru, aibou," he responded, stepping off of her.

"Eh…?" She pushed herself up, smoothing down her short skirt and the button-up top she was wearing. Ciel felt himself growing red. She was showing way too much leg…

Suddenly she caught his gaze. "Ah, dare ka, futaritomo?" Her gaze shifted between him and Sebastian, and then widened on his butler. "Moshikashite…"

"Eigo, Yako."

"Eigo?" She blinked. "You two speak English?"

"Yes," Sebastian responded stiffly.

"Ah, I'm sorry," she apologized with a pleasant smile. "I didn't realize."

"You would have if you had been observing, slug."

"Well excuse me for being focused on finding my dinner you stole, Neuro," she barked, rounding back on the other demon. "Speaking of which…"

"Oh, I suppose I can return it to you."

He presented her with a box writhing with tentacles, reeking of something Ciel did not want to figure out.

"Yappari…" she muttered, switching back to her native tongue. Neuro beamed, and tossed the box of ruined food over his head. "Well, my dinner can wait. It seems like we have more important matters anyway."

She approached them without any hesitation.

"Yako Katsuragi," she introduced herself, offering her hand to Sebastian. He faltered, glancing over to Neuro as though looking for permission. Ciel watched her raise an eyebrow, and then look over her shoulder at the blue-suited being. He gave her a pointed look.

"A handshake isn't going to kill me, Neuro. He may not be human, but I get the feeling he's not planning anything. He's protecting someone from you, after all."

"Human psychology is your field, not demons."

"You'd be surprised how many things we have in common, Neuro." Turning back to them, she sighed. "Since he's standing back there, he's not going to do anything."

"…I'll take your word for it," Sebastian replied, took her hand in his, and brought it to his lips. "Pleasure to meet you, Miss Katsuragi. I am Sebastian, butler to the Earl Phantomhive."

Yako flushed deeply. "Ano…"

"Sebastian, I don't think that's standard greeting in this… time period," Ciel observed.

"Oh, dear," he said with a light chuckle. "My apologies."

"No, no, it's alright. Just unexpected," Yako dismissed, quickly regaining her composure. "A demon butler? Neuro, why couldn't you be more like him?"

He scoffed. "To a human like you?"

"This human happens to be your partner," she replied with a deeply annoyed look. "Or at least I proved myself worthy enough for that title four years ago. You have a habit of forgetting that. On purpose."

"If I were one of his kind, your soul would have been devoured long ago. I'm surprised the child's soul is still intact."

"Eh?"

She whirled around, a look on her face that Ciel recognized as worry. He raised a hand to dismiss it.

"As per our contract," he began, stepping around Sebastian to regain something of his own commanding presence, "Sebastian has claim to my soul after he fulfils certain requirements." This was entirely strange to reveal to someone he hardly knew, but he had a strong feeling this woman would understand, given the demon she kept company with.

"But you're…" she started, and he knew without her needing to finish that objection that she was referring to his age.

"Fully capable of making my own decisions. I am the one who summoned him, after all."

They held each other's gaze for a long moment before Yako offered him a smile. He was struck with the sense that she understood him much more than their conversation should allow for.

"I see," she finally said. "Earl Phantomhive, wasn't it?"

"Ciel is fine," he corrected.

"Then please, call me Yako," she added, her smile widening. "I'll admit, I'm a bit curious about the contract you mentioned. Neuro and I don't have one." She paused, looking thoughtful, then worried. "We don't have a contract, right Neuro? I'm not grossly unaware of one, am I?"

He laughed, finding the question absolutely hilarious. The young woman seemed confused.

"Demons beyond a certain caliber needn't make contracts," Sebastian explained coolly, not sharing the other demon's glee. "The contract helps to maintain form and ability in the surface world for demons incapable of surviving otherwise. Without one, most demons dry up rather quickly."

This was news to Ciel, who looked between the two beings for a long moment.

"Is he… that much more powerful than you, Sebastian?"

Another fit of laughter from the colorful demon.

"Quite."

The simple answer left him surprised.

"I'm surprised to see you here, Lord Neuro," Sebastian continued. "The last I had heard, you were tackling the mystery of the Ninth Floor."

"It was a simple one," he replied with a disdainful frown. "I've since devoured all of the mysteries Hell had to offer."

Sebastian raised an eyebrow.

"Every mystery?"

"Did I stutter?"

"No, my apologies," Sebastian quickly backpedaled. "It is no wonder you have access to the Tools, then."

"I mastered them all long before I accomplished that task."

"I see."

"I don't understand a word of this conversation," Ciel interrupted. "And I do hate being left in the dark."

"Forgive me, Young Master," Sebastian said with a smile and a small bow. "We are speaking of the 777 Tools of the Demon World. They are tools available to only a select few residents, and even fewer master all of them. The eye we found earlier was one of those.

"There's the 7 Tools of the Demon Emperor, too," Yako chimed in with a note of curiosity. "The giant fish thing, and there's that tree, too. Evil Tree, right?"

"You have seen them?" Sebastian asked with wide eyes. His gaze quickly moved to Neuro. "You have been gifted those as well?"

In response, the other demon simply smiled widely.

"Why do I get the feeling that somehow makes him important?" Ciel and Yako said in unison, and then stared at each other in surprise.

"Important is an understatement. He is on par with the Emperor herself."

There was a long stretch of silence before Yako broke it.

"Neuro, I feel like you should tell me these things," she said in exasperation, this knowledge obviously new to her. "I've been working with you for how long now? Zera eluded to it when you went home, but eluding to and explaining are two very different things."

"You never asked," he said with an amused shrug. "Also, with me?"

"Fine, for you," she corrected. "Mattaku…"

A sudden, very audile growl echoed around the room, and all eyes fell to its source. Yako turned a very bright red from the attention. Her laugh followed the next rumble.

"I guess… I really need my dinner."

"Your stomach sounds like you haven't eaten in a year," Ciel commented. She rubbed the back of her head in embarrassment.

"It's been four hours, actually…" she admitted. "Why don't we move up to the office so we can discuss your situation more comfortably? I can have Godai drop by the convenience store, too."

"Office?"

"We're right below our detective agency," she elaborated. "Neuro solves the mysteries, but I'm the face of it. Keeps him from standing out too much."

Ciel and Sebastian exchanged looks. This was going to be interesting.

My knowledge and understanding of Japanese is pretty limited. I understand much more than I can write, SO, if someone more fluent has corrections to the dialogue, please let me know. I'm aware that commas are not a thing in Japanese, but I wanted the dialogue to be viewed from an English perspective, since Ceil understands none of it. Otherwise the following is the translation of each instance of Japanese:

What are you doing here?

(Give me) My lunch-ACK

You're interrupting (the conversation), partner.

Who are the both of you?

Is it possible...?

English, Yako.

English?

I knew it.

Um...

Good grief...

All of the crossovers (that I have read) for these series usually pin Neuro and Sebastian as equals or make Sebastian more powerful. I wanted to twist that and give Neuro the top spot for this, hence why Ciel and Sebastian are thrown into 21st Century Japan and not the other way around; it cuts Sebastian down another notch since he's not in his own territory. I used a little of my own headcanon here to make that make sense (see contract explanation and access to the 777 tools), and a lot of information from the end of the Majin Tantei series. More of that will show up in the next chapter.

This is set post-manga for Majin Tantei, and after the Cruise in Kuro.