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2025-12-01
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2026-03-14
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Undertale oneshot ideas I'd read as full fics

Summary:

So I've seen someone else do this and asked myself, why not? I have loads of ideas/situations for fanfics yet have no time to write them or I end up just procrastinating for two years after posting the first chapter (if you're looking at my other fanfic no you aren't). So here goes nothing I guess.

Some will be longer than others, some I might expand on and make multiple chapters, but I don't think I'll make them into full fanfics. Some will have more oc's or more lore/backstory and some won't have any. Some will be about specific skeletons, others will be about all of them. There will be different tropes, jobs and personalities for Reader, and I might make them into a bit of a self-insert sometimes.

Again, if anyone likes any of these, please feel free to use them! But do tell me, mostly to read the end result ;)

Notes:

This first chapter is mostly, but not exclusively, swapfell Sans (Black)/Reader.
I personally love when we have a strong MC, and is there anything better than an emotionally closed off Reader who has to be reached by none other than Mr. Emotionally constipated the skeleton?

When lonely souls meet, which one will be the first to break professionalism and admit they like each other's presence?

Chapter 1: Black & SAC

Notes:

(See the end of the chapter for notes.)

Chapter Text

It had been a few years since monsters arrived on the surface, maybe about two years after The Machine malfunctioned, resulting in the convergence of several universes. It had taken the monsters quite a while to figure out how to go on with their lives, how to make it work, find a place to stay and a more stable situation to go forward. They had made it tho, over a year later, but it was perfect: they secured a house big enough for all of them, with a few spare rooms just in case, they had all documents they needed to avoid suspicion, and monster rights were advancing.

Thanks to that last part, most of the household managed to go into the fields of work they desired. Black was no exception. He liked rules, law, like Razz, but found himself still drawn to physical work. Razz liked control and respect, but Black wanted more than that.

He still wanted the authority that came with a rank. He wanted to control and command. He was no lawyer, he was an agent.

He decided to join the only corps he found fitting for himself: ASEC. The Advanced Special Enforcement Command, a relatively new organization (established shortly before monsters appeared) in charge of both investigation and prosecution, and tactical operations. They not only dealt with building cases and arresting dangerous targets, they collected intelligence, ran undercover operations, executed raids, rescued hostages, and even responded to terrorist attacks. Right up his alley. Only issue was, no monster had made it inside yet.

But he wasn't going to let that stop him. And so he graduated, to the surprise of almost all his colleagues, but none of his ‘family members’. He had had a flawless performance his whole training, and the fact he had a recommendation letter from Her Majesty had helped as well.

He now stood outside the building, the shining symbol of ASEC looking down at him. He was to be given his first command today, finally meet his superior.

He went through the dark glass doors, standing in the hall, where he saw 2 or 3 people he had graduated with talking amongst each other. He would have approached them but he'd never really made conversation with them before, so he opted not to. But in the end, it seemed fate had decided they'd be talking, cause one of them called over to the skeleton.

“Yo! Black!”

He sighed, the casual tone of the Agent getting on his nerves. But he still turned to the voice and approached the group. Only a bit, no need to breathe the same air as them.

“Yes?”

“Congrats on the whole graduating thing, top of the class too!” Said a rather excitable young man, light auburn hair brushed back, gold earrings shining beneath the sun that came through the morning windows. Black couldn’t remember his name, but he did recall how annoying his voice had been any time the class would have to take notes and he wouldn't shut up “Heard you getting placed with a SAC, a tough one at that!”

Black knew that much already. He admitted, the prospect excited him. A SAC or SAIC was a Special Agent in Charge, someone who ran an entire field office, a bridge between HQ and the agents on the ground, the equivalent of a military colonel.

“We're gonna be placed with normal ahh agents so, tell us how that goes!” The man said, getting elbowed by one of his companions, who was side eyeing him. Whatever, Black already knew what people thought of him.

No matter in or out of the organization, no matter the place on earth, humans hated monsters. That's fine, Black hated humans back. He didn't think anything could change it at this point. He would help them get out of horrid situations other humans caused, and still see them blame his kind. His goal was the same either way, not to help humans, but to show them. Show them he was to be respected, heard.

His thoughts -and the conversation as a whole- were interrupted by a cough behind the group. Upon turning, they found the man who had been training them for the past few weeks, Chief Robb Doncaster. The man spoke with a raspy tone, no doubt from smoking too much, but everyone coped differently when it came to this life, so Black wasn't judging (now, if that was his brother…)

"Agents Mick, Audrey, Deon. Good luck today” He nodded, then looked down at the skeleton “Agent Black, please follow me.” He said, starting to walk down the hall, not bothering to look if Black had listened or not, because he knew he always did.

The walk was short and silent, not awkward, the air professional around them. Nothing related to the job to talk about meant not talking at all, and that was fine by Black. He wasn't here to make friends like some of his… counterparts would've. Actually, wasn't Blue starting his job after graduating the academy-

He'd have to ask later. Later. After doing this.

They went around a corner, yet another one. Finally a straight hall. The building was big, maybe reaching the level of a local hospital you'd find in the countryside. It wasn’t HQ after all. It was clean, organized, even silent, although sometimes they'd find agents chatting while taking a coffee during a break, their postures more relaxed than Black would ever allow his to be.

The doors by the end of the hallway were closed, and Black could tell this was their destination by the way the Chief's shoulders tensed -just enough for Black to notice-, and the way his feet slowed their practiced rhythm. He stood in front of the door, but before even knocking, a soft voice called from the other side.

“Come in.”

The Chief breathed in, and opened the door, slipping into the room, Black behind him.

It was an office. Actually, it didn't look like a private office. There were multiple tables, the ones you'd see used for police officers in the same department. Yet, only one of them seemed used, papers across it, still in an organized manner. In the middle of the room was a big white board, writing and pictures all over it. Some cardboards on the walls contained more documents. Only one human stood inside. They weren't looking at the new people in the room, their eyes set on the white board, their hip popped to the side, arms crossed, and a file in hand. Their brows were furrowed slightly, a nearly imperceptible frown adorning their face. They were absorbed in the case before them. A sight that sat just right with Black.

The Chief started “Ma'am, this is-”

They turned sharply, walking to them, stopping a few feet before the two.

Black took a better look at what he assumed would be his assigned Superior. They were taller than him, his head coming up to their nose at most. Still, they were quite short in comparison to the Chief, but that didn't seem to stop them from feeling like the most important person in the room, the confidence and authority they exuded filling the space.

"Is this my new recruit?” They simply asked. Their voice could be described as silky, but firm, it might've resembled Wine's, in the sense that both felt they could be used to convince you to do just about anything for them. Cunning. Intoxicating.

The Chief just nodded, Black thought for a second he might have been shuddering as he replied. “Yes ma'am.”

She just stared at the Chief for a second, before her eyes moved, looking right into Black's eyelights. Maybe even looking right through him.

She hummed, turning around. "Alright, that would be all. You can go, Doncaster.”

The Chief spared a glance at Black -was that pity?- before he exited the room. Leaving them alone. Mhh. Okay.

Black tried to assess the situation, but didn't get too far.

“Name.” She demanded.

“Agent Black… Ma'am.” he supplied.

She hummed “Put your things away” she pointed to a desk beside the one she was obviously using. “Come here when you're ready, I'll introduce you to our current case."


You were surprisingly direct. You had explained the case without having to read any of the files, knew it by heart and spared no details. Meticulous.

After explaining, you'd asked Black "What do you specialize in?"

He replied he could do whatever task you demanded. You just stared. Then gave him a rundown on what you two would be doing with that case during the week, explaining every step from a diplomatic point of view. "I trust you know the protocol, right?" You asked. He nodded without hesitation. "Good."

Given it was his first day, the other graduates and him were whisked away to a welcoming event soon after that, not allowing him to get any work done. When he managed to slip away back to his desk, his superior was writing away on her own, beside his. As if they were equals, how ridiculous. Your gaze was focused, you didn't look his way as he crossed the room, but did glance when he sat down.

"... what are you doing?"

He stopped in his tracks. What was he doing? Working, right? What was he supposed to say to that?

"Going Back To Work, Ma'am."

You raised an eyebrow at that.

"Are you skipping the ceremony?"

...

"Yes."

You didn't say anything, eyes going back to your laptop screen. "Go home then."

That startled him, did he do something wrong? He certainly didn't think he did. Was there anything else going on at the ceremony? Was it a trial? A test? He was going crazy overthinking, but you caught that and put an end to it.

"The welcoming ceremony, at the same time it marks the start of your duties here, also serves to show the last 'normal' day you'll have from now on. Last day without actively fearing for your lives, last day before forever keeping secrets from everyone you love, last day being able to stay away from your phones without the world going up in flames because you have one missed call. Might even be the last time you see the colleagues you graduated with before they're put inside a casket."

It was raw, but Black knew all you were saying was true. He wondered if that's why, during the little time he did stay there, in the ceremony, all he could hear about his SAC was how 'ruthless' she was. Regardless, he also heard how good she was at her job.

He couldn't help but wonder why he had been placed under you. Yes, he was top of his class but... a monster nonetheless. Maybe that's why. Experienced eyes watching his every move would ensure he could never mess up. And if he ever did, you could end him, throw him out. That's what the ASEC wanted, he was sure.

"So" you spoke up "If you're not going to enjoy that last good day between these walls, go home, and do it there."

He tried to argue he didn't need any more of a break. But you didn't bend. He ended up going home, to his nosy housemates as well. He managed to deviate the attention from himself to Blue, who had in fact just joined the Police Force, and was static to talk all dinner. 

But if there was someone Black couldn't avoid, it was his brother.

After all lights had gone out, Rus knocked on his door. Black sighed "Come In".

Rus opened the door just enough to step into the room, closing it behind it and throwing himself on top of Black's bed "so, sup bro?"

Black rolled his eyes "You’re Going To Have To Be More Specific Than That"

Rus tried again "how was work?"

Black sighed "Confusing."

Rus' brow raised in a silent question. Black sat beside him, and talked. Rus listened, like he always did, storing the information he received, and offering to turn himself in the following day to help Black get brownie points. After the suggestion, Black undoubtedly kicked him out of his room, but the almost imperceptible smirk on his face betrayed the fact his brother had helped him, just by lending his metaphorical ear.


The following day was better. Actual work like his superior had promised-

It was midday when he realized... he hadn't gotten your name. Not that he needed it if he was going to refer to you as Ma'am the whole time but... 

Luckily, a few days after the welcoming ceremony, you received a call.

"This is Y/N-"

He tuned out the rest of the conversation. Y/N. The name stayed on his mind for longer than he wanted it to. Let's see where you lead me, Y/N.

The days passed. First a few days, then a week, then two weeks. It was mostly paperwork, building cases not unlike a police department would, only with more sensitive information. 

Black took notice of how hard it was to read you. He was used to reading people, it shouldn't be this bad but... it was. Your eyes didn't reveal any of your thoughts, and you never gave any clear signs of emotion, of any emotion. Or, at first you didn't. 

It was hard to read you, but after a few weeks, Black noticed how your eyes had dark circles under them, how you'd stay for 16 hour shifts every two days, burried yourself in work. You were extremely good at keeping composure even there, but occasionally, barely, he could see that slip. A grunt here as you got irritated with a sloppy report, or a cut off yawn as you went to grab another coffee.

It was during one of these days -you had even stayed the night in the office- when Black was, for the first time in a while, pleasantly surprised.

You were reading over a file, writing notes over it, the coffee beside you hot enough that Black could see the steam rising above it. You rubbed your eyes and he caught sight of that too.

"Black" you called.

He straightened his posture, snapping his attention back to his desk. He replied without looking up. "Ma'am." The tip of his finger tapped against his keyboard, a tiny anxious tell that you didn't mention.

"Uhm" You took a moment to gather your thoughts. "I'm requesting your presence for a raid next week, if you're okay with that." You said tired, but even if you didn't seem to make a big deal out of this, it was his first real role outside paperwork all day and occasional patrols through the streets, the start of his actual work record.

His anticipation lasted all day. That night, Rus had to stay for longer than usual listening to his big brother, with shining starts for eyelights.


He started counting the days before the raid. 

5 days before it. 

You had been called to talk to the Deputy Director, sighing as you went back. Black dared to ask.

"Trouble, Ma'am?"

You just sighed again "It's fine."

You weren’t the only one called to talk that day, as a new cadet came through the door, maybe an hour later. You payed him no mind as he spoke.

"Uhm, someone is here for... Mr. Black?"

You raised a brow at that, curious, but Black had no clue who it was either.

"Th-they might be family-? of yours?"

Upon seeing he was making no move to go check, you spoke up "You can go check if you want."

He excused himself with a curt nod and went to the main hall, clicking his tongue once he saw the two familiar figures near the entrance.

"Edge, Red. What Are you Two Doing Here?"

Edge stood tall, his booming voice echoing through the halls "WE HAD BUSINESS HERE, RAZZ SENT US"

"yea', thought we'd call ya in case ya would 've extra info" said red

Black ended up agreeing to help them out, without going far. He was still on thin ice just by being a monster, after all. As they wrapped up their conversation tho, the crossover of the century just had to happen.

"Black" said your voice, steps approaching.

He turned sharply at the sound of his name. "Yes Ma'am?" Red raised a brow at that, looking over at you. Edge sized you up as well.

You just approached, glancing at the other two skeletons, giving them a simple "Hello, gentlemen." Turning to Black with a file in hand, asking him for things related to the case.

He took the file from your hand, his eye lights scanned the marked section with precision, mind already cross-referencing data. "Nothing New Was Logged Overnight, But I Ran A Trace On The Secondary Alias This Morning. Came Back With A Partial Location Ping, Abandoned Warehouse Near Dock 9. No Activity Reported Yet, But It’s Suspicious Given The Timeline... I Didn’t Flag It Yet. Wanted Your Call On Surveillance First."

Behind him, Red made exaggerated teasing faces at Black, Edge rolling his eyes and shoving his brother to the side. But Black's focus never left you.

In turn, your focus didn’t leave the file. "Okay, great, what about the current status of the known relatives? We contacted the parents, elderly couple. Any behavioral or social changes after? How are they holding up?"

"The Parents Were Interviewed Three Days Ago By Social Services. No Visitors Beyond Routine Check-ins. But... The Mother’s Vitals Spiked During The Call We Made Yesterday. Anxiety Symptoms. They’re Not Telling Us Everything. I Requested Access To Their Security Feed Under ‘Welfare Check’ Protocol. It Hasn’t Been Approved Yet." He stopped, for just a second, but you noticed the hesitance. "...But If You Sign Off On It Now, I Can Have It Pulled Before Noon."

You just hummed, grabbed the file, reading over it one last time, before leaving without another word.

Once you were out of there, Black turned back to the other two. Catching Red's expression, he readied himself for whatever he was about to hear. "What."

"oh, noooothing. jus' y'know, who was that hottie? ya got a chick while on th' job, eh? no shame, i'd tap that"

Black knew Red wasn't serious -well, he probably was, his standards weren't very high and you definitely fulfilled those- but he knew he was mostly saying that to get a rise out of Black. The worst part was that... it worked.

Black’s eye lights flared with magic. He stepped into Red’s space, looming despite their identical height. "You Will Not Speak Of Her Like That." His voice was low, sharp like a blade. "She Is My Superior. A Professional. And If I Hear You So Much As Breathe Her Name Again With That Tone-" He didn’t finish the threat.

Edge snorted behind them as Red put his hands up in fake surrender. That irritated him even more, but he now remembered just where he was. He couldn't fight here, even insults to his own family might get him into trouble, so he dismissed them and went back to his desk.

He took a deep breath, resuming his work, but apparently his poker face wasn't as good as he thought. 

"That grip is a bit tight there, try not to bend the table" you said, eyes still fixed on your screen.

He apologized and you shrugged it off.


3 days before the raid.

It was late. Like, really late. 2 am late.

It might have been the exhaustion from working too much, but Black barely even registered it as you stood up and told him to follow. Where?

Out, apparently. Night patrol on foot.

At first, he thought it was weird. But after a while, it felt pleasant, how peaceful it was at night, alone with someone he... trusted to have his back.

Still, not knowing the streets you were taking him through made him tense, until he smelled the unmistakable scent of spices coming from somewhere nearby. "Heard you're into Mexican" you started, the firsts words you'd said after leaving the office. Your voice... softer than usual, casual even? "What's your favorite food, recruit?"

"Uhm, Burritos I Suppose." He said, voice uncertain. Wow. Just what were you doing to him?

You just hummed.

How he ended up eating the best burrito he's ever had, with his superior, at what seemed to be her usual go to during night shifts, at 3am, beats him.

But he wouldn't trade that for the world. Otherwise, he would have missed how much you shared of you that night. How friendly you were with the family who worked the restaurant. Smiling, actually smiling. Speaking Spanish with them -You were actually fluent in a few languages other than English, "useful for the job", you had said-.

He would have missed it when you let it slip past your lips... you were lonely.

And as you both went back, hearing every breath, every step, every movement you made, he promised to himself.

I'm going to cook for her.


1 day before the raid.

It was lunch time. He was home, eating with the rest, on the table. That's when his phone rang.

He saw it was you and stepped into the hall. "...Ma'am?"

He could hear the end of something you were telling someone before you realized he'd picked up "Yes, Black, hello. I was just calling to let you know we're moving the raid, we're going tomorrow, 1 pm, sound good?"

His spine straightened instantly. The shift in tone was immediate. No more hesitance. "Understood. 1 PM Tomorrow." A pause. "Are The Updated Coordinates And Team Assignments Uploaded Yet? I’ll Need Time To Review Entry Points And Comms Protocols."

"I'll be with you at all times since it's your first time, don't overthink it too much, you're competent, you'll do good." You said before talking to somebody else on the other side of the phone, and coming back "It's all up, yes, you should be able to access that information. I wouldn't worry about the team assignments, I'll explain as we go, but coordinates and entry points will be important, these subjects have a high risk of escape"

He let you know he had it under control, and you hung up. Once he went back to the table, the air around it had changed.

Rus leaned over it with a slight grin, clearly amused "... who was that?"

Edge chuckled maliciously as red replied "ya future sister in law, mutt". That seemed to peak everyone's attention. Blue and Papyrus' in one hand, but Razz and Wine's on the other, for vastly different reasons.

He managed to get out of lunch in one piece -barely- and prepare himself for the following day.


The day of the raid.

He got there early, and you rolled with it, introducing him to the team, people under your command. Corporal Mason and Officers Luke and Sylvie. In charge of the entry, comms, and securing escape routes and the secondary base respectively.

The raid went... perfectly fine. The commands you gave were precise, sharp, clearly thought through even if most were made on the spot. I mean, Black could've obviously done that himself -he reasoned- but seeing you do it still gave him this... feeling. It was... joy? Like... the opposite of envy... And when he saw you take one of the men by surprise and disarm him with a simple punch in the gut... Black felt more than just that.

That night, there were reports to fill out, but for some reason you let Black go early.

"We should cherish the times when we can go home early. This is one of those" You said. "You... might not get this often from now on, your duties are going to be more demanding every time."

That same tone again. The one you used after the welcoming ceremony. The one you used sometimes when tired. When you had dinner with him. Why did that tone make him want to...

He couldn't think straight, he agreed, going home. But he didn't miss the weight behind your words. Your duties are going to be more demanding. You were making it so that he would have more opportunities like this, a real career. You were making sure he had the same opportunities as a human, he wasn't fighting against this species handicap alone.

That night, Black was quieter than usual during dinner, pushing food around his plate. 

They all noticed

Red was the first to mention it, leaning over with a smirk. "someone's extra quiet tnight"

When the questions started, he managed to make it seem like it was about the raid they had that day. But after dinner, in his bedroom, Rus came over, curiosity in his gaze. He leaned against the doorframe with a knowing smirk as Black read over some papers "... so". Black kept reading.

"ya were real quiet tonight."

Still no response, only a tighter jaw. Rus pushed off and walked closer.

"c'mon. you never get this distracted over missions unless somethin' else is going on." A pause. "was it 'bout her?"

That smirk of Rus' could get to Black sometimes, like it was doing now. Still, Rus kept asking "what's she like?"

Rus half sighed "bro, you can talk about it, y'know. ain't gonna do you no good ta hide it when it's so obvious, even if you can't do shit about it cause she yer superior or whateva, let it out with me. not with red tho, ya'll never hear the end of it"

Black sighed. And he started. Once he started, he could barely stop. He said it all.

How professional, efficient, precise, controlled you are. How cold everyone thinks you are. How sometimes you'll get a look in your eye while looking at an old case file like it means something personal, or how sometimes you just sit there staring into space with tired sockets. How he thinks -knows- you're lonely too. How soft the corners of your mouth look while smiling -truly smiling-, the occasional sarcastic remark you make when your buttons are pushed, the warmth you bring the room by just being in it. Like a light, brilliant, guiding his path, fighting the darkness from it before it reaches you. All you've done for him. All he wants to do for you. All he wants to learn about you. You see him. Hear him. He wants to know you in return.

Once again, Rus listened to his brother patiently. His smirk turning less teasing as Black stuttered a few times while trying to find the words to express himself. 

Holy shit. His brother was in love.

And it wasn't a love that would go away easily. It only got more tangled.

When he finally saw you break, in private, after you nearly lost a hostage.

When you met Blue while working together with the Police Force, and Blue brought you home. The way you faltered at a friendly hangout, opposite to a work environment.

When Rus teased you, but ultimately found out more about your past than you had planned to reveal to anyone.

When you defended the skeleton group during an outing, since the usage of magic is strictly prohibited, even in most cases of self defense, all without breaking a sweat. Which, might have stayed on the other skeletons' minds that night, if they were being honest.

When you went on an undercover mission where you had to act seductively, the state that left him in, and the following realization that you didn't need to act that way to have him on a chokehold.

When you finally collapsed when sick, only around him, and he made sure nothing bad could happen to you.

When you finally kissed him, and he felt his soul sing in a way it never had before.

Notes:

Ngl, I have a ton of ideas planned for this compilation.
I wasn't sure which one to start with, but I started writing this one and just kept going, so guess this was the one (it's 7am help).

I might expand on this chapter a little bit, with some of the scenes from the end there. And maybe some additional info (why was Black placed with a SAC as good as you, mhhhh?????). But, for the most part, this scenario is done.

Hope you liked this one, see you next time!