A/N: My first time writing an Undertale fanfiction, and it's an AU story...Really wasn't expecting that. I find the AUs for this fandom really fascinating and cool, but I've only truly found myself getting seriously interested in one particular AU. Swapfell. For those who aren't aware it's a combination of the Underswap and Underfell universes. So, basically, everybody's more evil, like in Fell, and the characters personalities are flipped, like in Swap. I'm fascinated with both of these AUs, but for one reason or another I've never really gotten seriously drawn into either of them...and then I stumbled upon Swapfell, a combination between the two, and get absolutely sucked in. It doesn't make much sense, but who am I to complain?

Keep the AU in mind when reading this story, or else it will probably make absolutely no sense and be extremely out of character.

Sans was skilled at a lot of things, and he was very proud of that fact. He was skilled at magic and combat and, despite the fact that he only had a single HP, had made his way up to the position of being second in command for the royal guard. He wasn't just a mindless brute like many of the guards were though. Sans was highly intelligent, and more observant than others seemed to think that he was.

Sans noticed things about people. When he met someone new, he could quickly get a fair grip on their personality and goals, and not just because of his ability to read their SOULs. And once he actually knew somebody, he could see when there was even the smallest change in their demeanor and soon after deduce why. He saw it as a type of game or puzzle, but very rarely did he ever involve himself in somebody else's problems. He was intrigued and amused by them, and that was it. Why should it be his responsibility to take care of somebody else's problem just because they couldn't figure things out for themselves?

The one exception to this was his brother.

Papyrus was...interesting, to say the very least. When it came to emotional and mental stability Papyrus' normal was what Sans would consider odd and potentially dangerous in anybody else. Papyrus lacked the motivation and drive to do anything. He was strong enough to be in the royal guard, and smart enough to be a royal scientist, but he was too lazy to even consider looking into either as a career choice. Sans had long since tired of trying to drag Papyrus into making something useful of himself.

Sans had always been annoyed by Papyrus' laid-back and uncaring nature, but he never thought too deeply about it because that was just the way that Papyrus was most of the time, and he wasn't going to change. Sans could only think of one thing that his brother was actively interested in, and it was something that Sans quite enjoyed himself.

Their little sessions.

Sans was not a patient monster. He was the kind of person who knew what he wanted, and he wasn't afraid to take it. He lived for the thrill that came with dominating over something, somebody. Sans was powerful, and he just loved to remind people of just how powerful he could be, and Papyrus was his favorite toy/subject/pet. Sans could shove the mutt into its place again and again, and he would never tire of it.

And Papyrus, the useless little whelp, actually seemed to enjoy it, which Sans still didn't understand. It didn't matter how hard Sans would hit him, how much humiliation he could put Papyrus through, how often he forced him down with blue magic, just because he was bored, Papyrus would always come crawling back to him, practically begging for him to do it again.

And Sans loved the mutt for it.

Papyrus knew his place in the world, underneath the heel of Sans' boot, and he was content there. As far as Sans knew, it was the only place where Papyrus felt completely comfortable and content. It was odd, and Sans didn't understand it, but he got a kick out of it. Papyrus was like a complicated puzzle, and Sans did love to solve puzzles.

Sometimes though, Papyrus didn't act like himself...no, that wasn't right. It was as though Papyrus' bad traits, his laziness, his proneness to sulking, and most especially his tendency to forget his place and annoy Sans, had gotten worse as his admirable traits, his obedience, his desire to please, and even his humor, seemed to be pushed to the background.

Sans hated it when Papyrus got like this, which happened every few weeks or so, and he was tired of not knowing what was going on. Obviously, Papyrus either didn't know that something was different, or he didn't care and just wanted to keep it a secret from Sans, which meant that Sans would have to get a bit creative if he wanted to find out the truth.

Fortunately, Sans was pretty good at being creative.

Sans could always tell fairly early on when Papyrus was having an off day...well, more of an off day than usual. Papyrus was always a big sleeper and was often difficult to wake up in the morning, but when he was having a really bad day he was all but impossible to wake up, and it was a waste of time to even try. On these days Sans usually just left for work, same as he usually did, and then punished Papyrus for his laziness after getting home. He thought that maybe Papyrus just needed his space and he would feel better if he just had some time to himself.

And while Papyrus did seem marginally better by the end of the day, he had somehow dragged himself out of bed and even allowed himself to grin or joke a little bit, Papyrus also seemed a bit worse. He moved slowly, carefully, almost as though he was scared of making a mistake, but that couldn't be the case, because Papyrus never cared whether or not he made a mistake, because he wasn't afraid of being punished.

Still, Sans wondered why his brother moved so stiffly, almost as though he was hurt…

...No, that couldn't be the case. Sans had long since made it clear to everybody in the Underground that Papyrus was his and his alone. Nobody could be so stupid as to harm Papyrus when everybody knew how possessive Sans could be.

Then again, there were some very stupid monsters.

Sans had no way of knowing if somebody was actually hurting his brother. Not without being smart about it. So when another morning came when Papyrus refused to move from his bed and no coaxing could get him to budge an inch, Sans had a plan in mind. He left the house, same as he did everyday for work. He didn't make his way towards New Home though. The second he was out of the house and sure that nobody was watching him, he took a shortcut back into the house that lead him straight into Papyrus' closet.

This wouldn't be the first time that Sans had spied on Papyrus, and it most certainly wouldn't be the last. He liked to know just how his mutt acted when he didn't think anybody was watching him. This would be the first time that Sans had done it because he was concerned. Usually it was just because he was bored or jealous.

Sans guessed that there was always a first time for everything.

Because Sans was just in his brother's closet, he couldn't actually see Papyrus, but he could hear him very clearly. Even so, Sans hadn't expected to hear anything for the first couple of , because he thought that Papyrus was asleep. So he was surprised to hear Papyrus rustling around on his bed. Papyrus wasn't a restless sleeper. He always fell asleep in a matter of seconds and then slept like a rock for hours on end.

So why did it sound like Papyrus was having a hard time getting to sleep?

After a half an hour or so, much less time than Sans had expected to wait, but much longer than he had ever thought he would have to listen to his brother rustle uncomfortably in his bed, there was a change. Sans heard Papyrus groan uncomfortably and then it sounded almost like he was getting out of bed, which was even more shocking to Sans than hearing him struggle to fall asleep. Sans couldn't even remember the last time Papyrus had gotten out of bed on his own.

Papyrus always acted differently when he thought he was alone, but Sans had never noticed such a change in him. It was strange, and concerning...and a little exciting. Sans couldn't wait to see where this was going to go. He leaned against the doorway and listened carefully, because he just didn't want to miss a thing.

Papyrus shuffled around the room...that was about it. For nearly an hour Sans could hear Papyrus shuffle, pause, and then shuffle again. Sans could see Papyrus' movements in his head. The mutt would look for something in his room to do or a comfortable place to sit, and he would stay there for a few moments until he got bored, and then he would move on to somewhere else

Papyrus was restless and indecisive, and Sans couldn't understand why. As far back as he could remember, Papyrus had never been restless, and certainly not indecisive. Papyrus hadn't ever been the most decisive person in the world, but that was only because he prefered to give up all control to Sans. If Papyrus needed to make a decision though, he didn't hesitate.

Something was clearly wrong with Papyrus, and Sans desired more than before to find out what it was. Partially because he was concerned, and partially because the mystery that was his brother had just deepened, and Sans couldn't wait to solve it.

After some time Papyrus finally seemed to settle down and everything was quiet for a few minutes. If it was any other day, Sans would believe that Papyrus had just curled up on the floor and fallen asleep. He couldn't believe that that was the case today. Sans continued listening, even more carefully than he had been before.

Sans immediately picked up a sound that sounded all too familiar to him. He usually relished in the sound of steel on bone, but now the sound just sent shivers down his spine. He just didn't want to accept that Papyrus, his precious pet, his little brother, was doing...that.

With how much that Papyrus seemed to live for his punishments and beatings, Sans had suspected that something like this would happen someday, but that didn't mean that he had to accept it.

Sans scowled as he felt the rage build up within him. He was a possessive skeleton, and he didn't react well to damage that was done to his belongings unless he was the cause of it. He wasn't going to stand for it, especially not from his normally obedient mutt.

Sans summed a bone attack and hit it against the closet door, completely tearing it apart. He didn't think twice about the damage to the door though, he was far too busy focusing on the sight of his mutt kneeling on the ground, with a knife from the kitchen that Sans had misplaced months ago in his hand, nicked slightly into his bone.

Papyrus looked up in surprise, the look on his face was similar to that of a monster who had dusted his first monster and gotten caught in the act. Sans didn't give his brother the time to react or even think. Sans just took care of the situation the best way that he knew how, by taking complete control of it.

Sans used his blue magic to get a grip on Papyrus' soul and didn't even bother being the least bit gentle as he flung his brother's to the wall. Papyrus let out a shout of surprise and lost his grip on Sans' kitchen knife. It clattered to the ground.

Despite having been caught in the middle of what he had to have known was a terrible act, Papyrus was remarkably calm. He looked somewhat guilty, but Sans got the feeling he just felt bad that he had gotten caught.

"M'lord…" Papyrus trailed off, because even the stupid mutt recognized that there wasn't anything he could say to quell Sans' fury.

"What the hell were you thinking?!" Sans shouted. Papyrus didn't even flinch, which just infuriated Sans even more. Why would Papyrus be taking this situation so calmly? "What...why…" Sans clenched his hands into fists in a vain attempt to stop them from shaking. "Why?"

Papyrus was silent, but Sans still waited for him to speak. Papyrus knew that when Sans asked a question, he expected an answer. So Sans waited.

"I...I needed...to feel." Papyrus said in a voice that was so quiet that Sans barely heard him at all.

"Feel?" Sans scowled in disgust and confusion. "Feel what?" Could his brother be anymore infuriatingly vague?

"Anything." Papyrus said desperately. "I-I'm tired, Sa-M'lord." An orange tint came over Papyrus' cheekbones as he felt embarrassed about his near slip up. "I'm so tired of feeling so empty all the time."

Sans felt his grip on Papyrus' soul waver. "T-there has to be another way for you to feel something without doing this." Sans lowered his brother to the ground so that he wouldn't accidentally drop him if he lost his grip on his soul. "There has to be-"

"I'm sorry, M'lord." Papyrus sunk down to the ground and kept his gaze fixed on the floor. He refused to meet Sans' eyes. "I'm sorry, I didn't know what else to do. I'm sorry."

Sans pushed his frustrations and disappointment in his brother and himself down and put up the old, familiar persona that he and Papyrus felt most comfortable with. "Stop apologizing." Sans ordered in a firm voice. Papyrus immediately stopped his muttering. Sans walked over to his brother's bed where Papyrus' dark purple jacket with the furry hood had been tossed aside carelessly. Sans picked up the jacket and carried it over to his brother.

Papyrus didn't react to Sans being there, so he gently laid a hand on his face and forced him to look up at him, though Sans was careful to not be too harsh about it. "Don't apologize." Sans repeated. "If you're really sorry, prove it. Useless words will get you nowhere. If you're truly sorry, then you need to swear that you won't do anything like this again."

"I…" Papyrus whimpered. "...I'll try."

Sans grinned. "Good." He took Papyrus' jacket and draped it over his shoulders. Sans flipped the hood up so that Papyrus' face was surrounded by the fur, just the way he liked it. "Now you look more like yourself." Papyrus gave Sans a weak smile. It looked more like a grimace, but at least it was something. At least Papyrus was willing to try.

Sans knelt down on the ground in front of his brother and grabbed his injured arm. "Hold still." Sans ordered. Papyrus opened his mouth, probably to ask what he was doing, but he flinched and shut his mouth when Sans began to use his magic to try to heal the nicks in his brother's arm.

Sans was good at magic, but he had never been able to get a grip on healing. He could do it, just not very well. It didn't help that hard bones were much more difficult to heal than soft, squishy flesh or fur. Sans knew that he would feel drained after healing the nicks on Papyrus' bone, but it had to be done. Sans could see some older, little scars on Papyrus arms, probably from when he had done this before, and that wasn't acceptable. Sans could injure and scar his brother, but nobody else, not even Papyrus, was allowed to do that. Sans needed to get rid of all traces of Papyrus ever trying to harm himself.

And Sans knew how much Papyrus hated to be healed with magic, he prefered to let his bones heal naturally over time, so maybe this would serve as a lesson to keep Papyrus from making this mistake all over again.

"Don't do this again, whelp." Sans scolded. Papyrus relaxed at the familiar term. "You're making me look bad."

"I'm sorry, M'lord." Papyrus said, even though Sans had already told him twice now to stop apologizing.

"I'm being serious, Papyrus." Sans used his brother's actual name to show just how serious he was. Sans could feel his left eye begin to glow and give off magical energy because of his barely restrained emotions. "You're mine, and it's my job to keep my belongings safe. If I can't do that, then what kind of master am I?"

"A great one." Papyrus insisted, his right eye began to glow orange to match Sans' glowing red one. "I...I made a mistake."

"You didn't know any better." Sans finished healing the nicks in Papyrus' ulna and radius bones, but he didn't let go of his brother's arm. "It's alright though. I'll just have to teach you to be better."

Papyrus visibly perked up at Sans' words. They both knew that Sans' had two different methods for teaching Papyrus a lesson. He would either hurt and punish him for misbehaving, which Sans wouldn't do this time, because Papyrus had already hurt himself and hurting him further wouldn't teach him anything. Or Sans would spend more time with Papyrus, take him everywhere he went, so that his mutt of a brother could learn through example.

Sans knew how much Papyrus enjoyed these lessons, because he enjoyed the attention. He enjoyed not having the responsibility of doing things on his own, that his only responsibility during these lessons was to sit quietly and watch. Papyrus enjoyed being around Sans so much, and Sans enjoyed his company as well, even if he had to deal with the occasional confused guard member who tried to provoke Papyrus into doing something that he shouldn't. These watch and learn lessons were always very effective, even if they didn't always stick for very long.

Sans would make sure that this one did though. If he had to use a different approach or use these lessons for a longer period of time, Sans would make sure that this lesson stuck. He would be damned if he let Papyrus hurt himself again. If that meant that Sans needed to keep a constant eye on his brother and actually do something to keep his mental and emotional state more stable, then so be it.

Sans was willing to put in a little bit of effort if it meant he got to keep his toy, his pet, his brother, at his side.

A/N: Geez, their relationship is complicated as heck, and super fun to write. I'm definitely going to be writing more Swapfell sometime. I've actually been toying with this idea that also involves regular Underswap...well, we'll just see how this plays out.