Chapter 1: You Can Teach Demons New Tricks

"Ow! Must you be so rough?!"

"You tensed up and jerked away from me… If you had relaxed like I told you to, you would've been fine." The doctor grumbled, pulling the needle away.

"I still don't know why I need these yearly shots, it's so demeaning!" The patient complained, lavender eyes trying to burn holes into the doctor's back.

"You know damn well why you need these shots, Yushiro. Your blood is thinning. She only gave you as much as you needed to survive, and slowly it's being purged from your system."

The Demon known as Yushiro continued before the doctor could finish, "And when it's gone I'll be human again. You're stalling the process by giving me your blood, Datenshi." Yushiro glared at the now named doctor, but it lacked heat.

"And so far it's working. You should've died a century ago, and yet here we are."

"And we have no idea when she's coming back," Yushiro lamented. There was silence between the two of them. "Usually this is where you say something dark or hopeless, Doctor."

Datenshi sighed, typing something at his desk. "Yet nothing comes to mind…" Yushiro rubbed his already healed arm and stood. "I want her to come back too, you know." Both of them stopped, looking down.

"I miss her…" Yushiro whispered.

"I know… I know…" The doctor sighed before looking at the centuries old artist. "But we have each other, the last of our kind."

Yushiro scoffed. "Please, you being soft is making me sick."

Datenshi huffed, shaking his head. "I thought I was going to vomit just saying it. Get out of my office you ungrateful ingrate, I have patients to see."

Yushiro growled before fast walking out of the door, pausing just before he disappeared. "Thank you…" he said sincerely before vanishing from sight.

"Anytime, my old friend…" Finishing his document he stood and entered the lobby of his clinic. For a moment he stared out of the window, beyond, and through the thick viewing port across the small walkway. Five gigantic metallic arms extended away from him. Each of them were gargantuan cities of their own. Beyond? A purple hued nebula with hundreds of space faring vessels going to and from the giant station.

"May you live in interesting times indeed." He muttered to himself before a mother and daughter rushed through the Clinic's entrance. His eyes momentarily lingered on their blue-hued skin before he got to work.

"My daughter!– Her arm…" The mother stressed.

He gently took the arm of the little girl, maybe five or six, and gave it a brief inspection. "It doesn't appear to be anything serious, but please follow me." The three of them went back into the room he had just left, leaving the view alone.

May you live in interesting times indeed…

"You, kiddo, are a very lucky little girl." The doctor said, looking at the screen in front of him.

"Will she be ok?"

The man looked up at the mother, a woman with blue skin and a curved crest in place of the hair that he was used to. This woman, and the child he was examining, were both Asari. Asari were a mono-gendered species of humanoid aliens resembling human females, albeit with a crest and blue or sometimes teal skin. And like he had already noticed; Aliens. But such was life on the citadel, and the life that he had mostly gotten used to these past twenty some years.

"Yes," he said after a moment, "she'll walk away with a sprained wrist and minor bruising. Which is remarkable considering she fell from a second story window. You said that she used her biotics to stop the fall?"

"Y-Yes, right before she hit the ground she used what looked like Lift on herself. When she stopped it she landed on her arm." The mother said, still visibly distressed.

"I see. Well then…" The doctor grabbed some painkillers from a small safe and walked back to the counter. "My recommendation is that she take it easy for the rest of the day. In humans a sprained wrist usually takes about one to three weeks to fully heal, so because of our similar body structure I'd say that'll be pretty similar. Try not to move it too much, I'll be right back with some simple painkillers.

As he turned to walk though the little girl spoke. "That man isn't normal mommy."

The mother had a look of mortification on her face but the doctor just smiled. "You're right. I'm a doctor." And he went to get the pills, frowning slightly when neither could see his face. He stopped in front of a mirror inside of the storage room, muttering. "What gave it away? The eyes?" No, the contacts hadn't slipped, they were still brown. "Fangs maybe?" He bared his teeth and chomped twice before making a noncommittal hum. The face that looked back at him was a lightly tanned man maybe in his early thirties, he had brown eyes that were usually a yellow-orange, and a somewhat angular face. His dark brown hair was combed over and neatly trimmed. Without anyone to see him, his face was cold and his eyes lacked any warmth.

He walked back out to the family and the suspicious child. He had a plan for that though, one that always worked. "Here ya go, a little tradition human doctors have." He handed the girl a lollipop, and the suspicion vanished in seconds. Oh the sweet innocence of a child. Give them candy and all their woes disappear. "Because of how young she is I also recommend getting something to eat. Using her biotics when she's this young will tend to take quite a bit out of her. I think Ryba's is still open around this time? Oh, and I wouldn't be surprised if she calls it a night earlier than usual as well." He told the mother as she took the small bag he packed for her.

"Thank you so much, Dr. Datenshi! You've been wonderful."

"Of course, have a pleasant evening." Datenshi smiled as he watched the mother and daughter walk away. He wasn't lying when he said the child was lucky, she had her whole life ahead of her. All one thousand years of it. And even if the two of them never met again he'd be around for all of it… And her daughters… And her daughter's daughters… Because Datenshi wasn't a human. Not anymore… He lost his smile.

Datenshi was a demon. He'd been a demon for the past five hundred and fifty some years. He was around when his old master almost died, and he had been there four hundred years later to make sure he did die. He was around for the greatest generation of Demon Slayers, and one by one he watched every single one of them pass on. The same went for their children, and their children's children. All the way until now. In fact, they were one of the reasons he was here at the Citadel rather than Earth. After the First Contact War, a slew of humans moved to the center of galactic civilization. One of those families were the descendants of Tanjiro and Kanao Kamado. They lived an uninterrupted and peaceful life, and had even gotten rather famous due to their family's Kagura dance. Every New Year the family performed the Sun Breathing Dance in the Presidium, and over the years it had evolved into an almost unofficial holiday. People from all over would come to watch the spectacle, and it even had multiple recordings on the extranet.

Of course, like their ancestors' personalities, the family refused every option to monetize the event or charge some kind of fee for admission. They were just performing the dance. The fact that people enjoyed it brought them joy, but in their minds it would be disrespectful to their ancestors to demand money for it. It was a family tradition, not a performance for money.

Ever since they moved here, Datenshi hadn't missed a single performance.

The rest of his day went by slowly and the Demon found himself lost in the monotony of his daily work. Five scheduled appointments, two consultations, one unplanned examination, one break to… replenish himself, and an hour or so to take stock of what he had before closing up. Without his assistant here it was even more boring than usual, but he could hardly complain. The early 20th Century had been… Well, he could easily claim to have experienced his fair share of 'excitement' to say the least. He was more than content with his boring day. At least…

Thud-Thud-Thud!

His eyes snapped to his door. Somebody was banging on the entrance, rather desperately in fact. He keyed his omni-tool and the door slid open, revealing a—

"Are you a doctor?! I-I, Please! If-If you could give me some medi-gel I'd be more than happy to leave!"

It was a young Quarian woman. She was wearing the suits that their entire race was confined to with purple fabric added to it and around her helmet. He barely paid attention to any of that, because the young woman was holding her left shoulder, and if the liquid under her hand and sour smell was anything to go by she was wounded.

"Come in and sit down. Have you already treated yourself or did you come straight here?" He quickly stood up and rushed to his wall locker, taking out a canister of medi-gel and a roll of surgical bands.

"M-My suit isolated the wound and I already gave myself antibiotics but I couldn't do anything more than that!" She stood next to one of his beds, still cautious and wary. It was understandable, if a bit annoying considering treating jumpy patients rarely ended well.

"That's good, makes things a lot simpler. I promise you that I'm a friend, if you could sit down this will go by much faster. I need to examine the wound and make sure you weren't hit by a weapon mod, and when I apply the medi-gel it's going to sting." The young Quarian still eyed him warily. "You can trust me," he said earnestly. She stared at him for a moment longer before slowly sitting down on the bed.

He brought up his omni-tool and scanned the wound for any abnormalities. Luckily there weren't any. "It wasn't modified. The shot went in and out, and didn't even graze a bone." With that done he applied the medi-gel as gently as he could, pausing when she flinched away. "By the way, I never caught your name, miss?"

She was quiet for a moment, "Tali… Tali'Zorah nar Rayya…"

"Nice to meet you, miss Zorah. You can call me Dr. Datenshi." He gently tightened the medical band around her shoulder, making sure it wouldn't move or fall off. "Medi-gel works fast, so it shouldn't take more than an hour or two to fully heal up, but agitating the wound too much may cause it to tear open again. So I'd say keep the band on for about… three or so hours before removing it."

He stepped back and Tali rolled her shoulder before looking at him with her slightly glowing eyes. "Thank you, Dr. Datenshi. I wasn't sure if…" The rest went unsaid, but beyond that there was more she wanted to say. Quarians could be difficult to read, but he didn't need to, not when he could smell the uncertainty coming from her. "You've already done so much, but…"

Slowly the Demon in disguise took a seat on the bed across from her, folding his hands and leaning forward slightly. "I'm here to help. If you need something then just ask."

Tali wrung her hands together before she sighed, "I recently came across some important information. I… have proof that the SPECTRE Saren Arterius is in league with the Geth." Datenshi narrowed his eyes, but listened regardless. A SPECTRE working with the Geth seemed… unlikely. "But people are after me, they want to kill me because of it. I-I… If I could get in contact with the Shadow Broker I could trade the information for a safe place to stay."

Dr. Datenshi narrowed his eyes in thought. "There are a number of informants all over the Citadel, but I guess the closest one would be Fist. He runs a nightclub called Chora's Den not too far from here. I'd be careful though, he's an egotistical womanizer with a record for gang violence." He saw the determination in Tali's eyes, she had latched on and wasn't going to let go. He sighed, "however, he's probably your best bet."

"How do you know him?" Tali asked, seeming genuinely curious.

"We're not friends if that's what you're asking. Sometimes if I need a rare medicine or something that's… somewhat less than legal I can usually get it from him." He explained.

She tilted her head, narrowing her eyes. "Less than legal?" she parroted.

"Nothing dangerous. The Citadel's laws on medicine can be somewhat outdated, there's a number of human medicine and drugs that are blacklisted from Citadel markets, and sometimes those can be the difference between life and death. I won't let a patient die because of outdated laws." The doctor and Tali talked for a few more minutes before it was time for her to go. When she asked about payment he waved her off, nothing he used was hard to replace and he had a surplus of it all anyways.

"Before you go," he called out as she opened the door. As discreetly as he could he handed her a small disc. "A concussion grenade," he explained. "You said people are after you, and when it comes to the Wards anything goes."

"I thought the Citadel was supposed to be safe?" asked the wayward Pilgrim.

"Usually it is, but this isn't the Presidium. Who knows what could happen?" Tali gripped the grenade before nodding. He nodded back, "safe travels." Without another word the Quarian disappeared down an alley. He hoped everything went well for her, because something in his gut told him something was off. In his… experiences, his gut had never led him astray… Oh well, he was under no obligation to do something about that. Whether or not the girl would be fine was out of his hands, and besides, it wasn't like he would be seeing her again any time soon…


Unfortunately after a few days of blissful normalcy, his life became very complicated, very quickly.

"EDEN PRIME ATTACKED BY GETH"

Skimming through the article made him frown. The Geth, a sentient race of machines that hadn't been seen for three centuries, had launched a massive raid on the colony of Eden Prime. Civilian losses were staggering and the Alliance Garrison had taken heavy casualties. Colonial Investors were pulling funds from future projects and colonial registration had dropped sharply. None of it affected him, but it was never something he enjoyed seeing. The Geth though? Was that why Tali was so desperate and secretive about her information? The news article was hours old at this point so it had probably circulated across the extranet already. He was just old fashioned and rarely kept up with current events. There was something else about a Turian SPECTRE but he was interrupted by more banging at his door.

"Tch… If this isn't an emergency then please consult our business hours!" He called out, already putting the interruption from his mind. But the banging on his door only grew more intense. He barely controlled his growling, and vaguely he could feel his fangs elongating ever so slightly. After another series of banging at his door he swore under his breath and opened the door.

"Unfortunately I don't deal with hearing problems! This clinic is closed, so—" A human male jammed a pistol against his throat and shoved him against the wall. Three more humans entered, all armed, and the last one closed the door behind him. All of them were wearing red clothes, they were Fist's men?

"That Quarian told you everything, didn't she?" The man holding him at gunpoint asked.

"I don't know what you're talking about, now put down your oversized toy and get out!" Datenshi growled, furious.

"Don't play coy with us, we know she came around here, and we know you treated her!" The other three surrounded him, smug and confident. A vein popped on his forehead. These people had no idea… Oh, it would be so easy! One jerk and the man in front of him would lose an arm, a swipe of his hand and his head would be next. He had made a vow not to kill anybody unless it was absolutely necessary, but man if he wasn't tempted!

"What's it matter to you? The things discussed between doctor and patient are confidential!" He gave the men a scary smile, eyes burning with hatred. "But I'll use small words since you seem to be mentally challenged. I'm not telling you a fucking thing!" The door to his clinic opened and he saw a familiar blue armored Turian enter. The C-Sec officer immediately took note of what was happening and pulled his pistol before crouching behind his desk. The goons were too stupid to realize the door had opened.

It was nice of Garrus Vakarian to lend a hand, the Turian had been nothing but cordial when picking up supplies, but honestly? His Pride as a Demon demanded that he turn these wretches into stains. The hidden Kanji carved into his eyes burned.

The other goons surrounded him, forming a triangle. "I thought doctors were supposed to be smart. Lucky for us you're nothing but a loose end~" He resisted the urge to scoff in annoyance… And with Garrus here he also resisted the urge to turn the man's head into a bloody pulp. His day continued to get even more complicated, as three more armored humans walked through the door of his clinic. Two women and a man. All three pulled out their weapons upon seeing the situation.

One of the goons noticed the new arrivals. "Hey!"

The man holding him "hostage" jerked him around until he… Oh no… "Who are you?!" There was no way that Datenshi was being used as a human shield… This time his annoyed scowl did show on his face.

"Let him go!" The leader of the trio yelled. The woman was wearing a set of light black armor with a red stripe down her right arm. She had fiery red hair and blazing emerald eyes that held a kind yet analytical gleam to them. She looked at him and gave a reassuring smile. "You're gonna be alright, doctor."

"Great…" He said dryly. Garrus took the distraction and leapt out of hiding. Before he even slowed down he shot Datenshi's captor in the head, killing him instantly. The new trio didn't waste a second before opening fire on the remaining goons, and before his captor's body even hit the ground it was over. Honestly? He was impressed.

Garrus turned to the new arrivals. "Perfect timing, Shepard! You gave me a clear shot at that bastard."

The redhead, Shepard, wasn't as impressed as Datenshi was. "What were you thinking?" She asked with a frown, "you could've hit the hostage!"

Garrus looked taken aback at the accusation, "I– There wasn't time to think. I just reacted– Dr. Datenshi, are you alright?"

"Other than being annoyed? I'm fine thanks to you four," he lied. He was never in any danger, but the people who knew about his true nature were very few, and not currently in this room.

Shepard walked over to the two of them. "I know those men threatened you, but if you know who they worked for we can figure all of this out." She spoke to him in a concerned manner.

"Ma'am, your concern is appreciated but I'm not nervous or shaken. This is hardly my first "Thrashing."" He said, using air quotes at the end, "and they work for a man named Fist, who has just made his way to the top of my Shit List…"

"What were they looking for? I doubt they came here for no reason," the human male asked. Datenshi narrowed his eyes slightly, they were looking for something…

After a moment he sighed. "A couple of days ago a young Quarian came to my clinic. She had been shot in the shoulder, and from what she told me she was running from somebody. She wanted to get in contact with the shadow broker… Trade some valuable information for a safe place to lay low, so I told her to get in contact with Fist. He's a complete scumbag but he's the closest representative."

"Not anymore," added Garrus, "now he works for Saren, and the Shadow Broker isn't happy about it."

Datenshi resisted the urge to growl. That's what his gut was warning him about! And he had sent Tali straight into the lion's den. "If Fist wanted to reduce his life expectancy then he definitely succeeded."

"That Quarian must have something Saren wants! Something worth crossing the Shadow Broker." Garrus guessed.

"You're not wrong. She told me that the information had proof that Saren was in league with the Geth," Datenshi cupped his chin. "And the Geth recently attacked Eden Prime… Would I be correct in guessing that Saren was behind the attack on Eden Prime?"

Shepard crossed her arms over her chest. "The Council's protecting him, and without concrete proof they won't do a thing about it."

"Yeah that sounds like them alright…" He grumbled. Sparatus would always take the Turians' side. Valern would always be the skeptic. And Tevos would sometimes play devil's advocate, but she'd usually side with Sparatus' decision unless it went against what she believed in. From what he had witnessed, the Council was… flawed, to say the least. "Well if you want that information you need to get moving, the quicker the better. I sent Tali straight to Fist so she's walking right into a trap."

Garrus turned to Shepard, "This is your show Shepard, but I want to bring Saren down just as much as you do." Datenshi tuned out the rest of the conversation as it didn't particularly involve him, but he piped up as they turned to leave.

"If you need medi-gel feel free to restock here, and if you get injured I'll patch you up free of charge. Do try to stay alive? I don't enjoy owing debts to dead people, it causes complications…" He wasn't exactly sure where hell was, but if he knew he'd be glaring at Tamayo. As much as he missed her… He shook his head. The quartet left his clinic in a hurry. Apparently they were making a detour to pick up a Krogan bounty hunter. If they succeeded, their chances of survival would increase exponentially.

The demonic doctor looked around his clinic, noticing the new bullet holes and pockmarks littering the walls. With a sigh he grabbed a canister of omni-gel and got to work fixing the damage.


COMMANDER JANE SHEPARD, FIRST HUMAN SPECTRE

When he sent Shepard the same way he sent Tali, this was not the expected outcome. He was about a third of the way through reading the article when his door opened. The person who came through was… not who he was expecting.

"Karin," he said, eyes going wide. "This is a pleasant surprise, what brings you to my corner of the Citadel?"

"Well as much as I wish this was just a common courtesy call, unfortunately that's not the case." Karin Chakwas was an older woman wearing the Alliance's standard medical scrubs. She had graying silver hair and kind green eyes. The woman had been a close friend for many years, and one of the few he actually tolerated. Being a demon with friends… He was always left behind… Right now was the perfect example, what passed for his heart clenched as he saw new wrinkles and more gray than silver. It had been years since they had seen each other in person.

Apparently he wasn't able to hide his look, because Karin just smirked. "Now, now~ I believe we've had this conversation before."

Datenshi sighed, "I shouldn't mourn time lost, instead I should be happy for what time we had. I know." A small, genuine smile appeared on his face. "It's good to see you."

She returned his smile. "Likewise."

Unfortunately though, "But as nice as our reunions always are, we should probably discuss the reason you're here."

"Agreed…" Karin took a seat on one of his medical beds, frowning slightly. "It's time…"

The demon sighed before sitting next to her. "I'm sorry. I know you've been dreading this for years."

"I've already cleared it with Anderson and Hackett, Eden Prime was going to be my last run. Unfortunately I'm too old to go galivanting across the galaxy." She was silent for a time. "I… don't want to go…"

He stared straight ahead, momentarily lost in thought as a multitude of faces flashed before his eyes. "Nobody ever does."

Karin let out a huff of amusement, "There are times when I'm jealous of you, you know? I know it's foolish, but the more I age the more I find myself wishing for more time." Sometimes he forgot that she was one of the few people that knew about his true nature.

"I know you, Karin. You don't regret any of it… And don't you dare go off about feeling like you "abandoned the Alliance." You've given the majority of your adult life to them, if there is anybody who can say they deserve to retire, it's you."

"I could say the same thing about you getting some rest." She muttered.

Datenshi barked out a laugh, "I'd go stir crazy within the first week." The two of them shared a laugh before falling into a comfortable silence.

"But I will feel somewhat bad about what comes next." He raised an eyebrow at her statement. "Anderson is giving Commander Shepard the Normandy in her hunt for Saren. Unfortunately the ship just lost its CMO, and the Alliance will take too long sending a replacement."

"Karin…" He said slowly.

Before he could continue she cut him off, "I'm calling in that favor you owe me."

"Absolutely not!" Datenshi stood from the bed and looked at the amused woman like she was crazy.

"You can't refuse the favor, remember the promises we made?" He did remember, he remembered them explicitly…

"I'm technically a civilian."

"Shepard is a SPECTRE now, she can bring whoever she wants on this mission, given that they come along willingly."

"I'm a Demon…"

"And you'll be on the Normandy most, if not all, of the time. No sunlight, no combat, nothing that'll give you away." She had a smug smirk on her face. She had him and she knew it. He wouldn't say no, and no matter how much he would glare he didn't have it in himself to hate her for it.

Datenshi let out a resigned sigh. "You might as well call the Commander over so we can explain what's going on. Minus my true nature, of course…"

"Of course." The aging doctor brought up her omni-tool and called the commander, telling her to come to his clinic.

Dr. Datenshi crossed his arms and sighed for the umpteenth time. May you live in interesting times indeed…


Spain, Mid-16th Century

"Brother Alexandre."

"A moment, please."

The Jesuit doctor held up a hand before focusing on his patient. "Take a deep breath for me?" The man complied. "I've dealt with Plague victims before, and I can confidently say that you are suffering from a light cold and nothing more. I wouldn't worry, but if the coughing persists I or another doctor can give you cough syrup." The man thanked him profusely before leaving his small room.

"I apologize," Alexandre gave his new visitor a once over, "Father, but I find it discourteous to ignore those in need."

The man smiled and gave a nod, "No apology is necessary. The Lord has gifted you with special talents, I would be doing you a discourtesy in preventing you from working." His visitor had neatly trimmed hair and a well groomed beard, and had kind intelligent eyes. "If you're not busy could I borrow a moment of your time?"

Alexandre gestured to a small chair and took a seat himself, "I have nothing pressing. That man was my last patient for the day, unless someone has an accident I have all the time in the world. But before anything you appear to have me at a disadvantage."

"Of course, my apologies." The man placed a hand over his chest and grinned. "My name is Father Francis Xavier. I was wondering if I could share an opportunity with you?"


Well this was a completely unexpected idea that came out of left field. In reality these are two universes that should not be able to mix, but I've found a way to make it work! A lot of this story has already been planned out and serious thought has gone into this to make it feasible. As you've seen the main character is a Demon, and more information will be revealed as time goes on~ And if the ending was any indication I'm going to be alternating between his backstory and Mass Effect, which means we'll be seeing the canon events of both!

So without further ado, I present to you the first (at the time of writing, October 7, 2023) to my knowledge; crossover between Kimetsu no Yaiba, Demon Slayer and Mass Effect! If I am incorrect I will happily give credit the next time I post a chapter.

Because this is a new story, for those of you who don't know me, welcome! I hope you guys enjoy, and if you'd be so kind I'd love it if you checked out my other works! It's great to have you here, and I'm sorry in advance for my wacky ideas~

For returning readers, welcome as well! I've been teasing this story at the end of my most recent works and it's finally here! This is going to be added into my rotation between The Dragon and the Fox and Mariana, the Abyssal Hero! Although because I'm writing two stories at once in a way, I may skip over a chapter here or there and move its spot. Obviously, you'll see which story is next so don't worry too much. I hope you guys enjoy!

That's all I have for now, I'm not going to spoil any ideas yet so I'll leave it here and we'll see where this goes!

Next in the Rotation: Mariana, the Abyssal Hero!

Thanks for reading and I'll see you all soon~