AU Sans X Reader Long OneShots

Underfell Sans X Reader Mercy not Spare

You are a human who fell down into the underground you haven't killed anyone, but you have come close. You refused help from flowey and Toriel. You have built you reputation as someone not to mess with. But you don't want to hurt anyone, on the inside you a kind and caring person in an unforgiving world.

I walked through the cold to my home in Snowdin. Monsters challenged me all the way, but then a gang of dogs monsters jumped me, "So your a human, huh, you think your so tough." That word sent hatred through my thoughts, If you're not Tough here, you'll be dead before you can object. I dodged as the leader of the dogs slung his swords at me. I jabbed at him with my knife, only to be struck as well. I grabbed my arm where a dog named Doggo had sliced it. I sent my knife into his chest lowering his hp to 3. Doggo ran off whining.
The rest of his team soon followed him as I fought back. I fell to my knees holding my arm, it was sliced to the bone. "Heh wow kid you look like you could use some help." A skeleton in a black hoodie walked out of the trees. I stood up on one knee, and held my knife out towards him. "Keep your distance skeleton," I spat, "I can take you." The skeleton looked unamused, "My names Sans, what about you." I stood up fully the knife now at my side still ready to strike. "Y/n. So uh why aren't you killing me?" Sans looked at me, "Well. . . You don't really look like the type to kill." I growled, "What makes you say that?" He chuckled, "Kid I'm just sayin you don't want to kill and neither do I, so let's be pals." The skeleton held out his hand, Why not everyone here wants me dead anyway. I took his hand feeling the world spin, "Oh shoot, What the heck." The world stopped I stared around wide eyed. "You-You can teleport." I stared at the skeleton, "Ya, what." "So why are you being so nice to me?" Sans paused, "Well, your the first person I've seen use spare in awhile." I cringed, "It's not spare it's mercy, killing off anyone down here won't help us escape. I looked at Sans, "So, why'd you drag me here?" Sans turned, "I'm gonna make you a deal, you pretend your my human, and I won't let anyone kill you." I paused, "Excuse me?" "Like if anyone try's to mess with you, if they think your my human they'll think twice." I grimaced, "What do you want in return?" Sans laughed, "Ha I thought you wouldn't ask,  . . I want you to teach me how to Spare." Sans sounded unsure about how to use the word. I laughed, "You expect me to believe that load of bull. Do you take me for an idiot?" I couldn't believe my ears, This skeleton thinks I'm stupid. No one in this universe uses spare. The skeleton gave me a sideways grin, "I swear to ya on my life." Really he's going to swear his life. Here if you swear you life and the person breaks the swear, you can kill them or really whatever. "Alright whatever," I didn't want to do this but right now I didn't really have a choice, "But by the way it's mercy not spare."
Me and Sans chatted for awhile and told jokes back and forth. At first I didn't really trust this guy, but I was warming up to him. I really liked being able to chat with someone without the threat of being killed for it.  "So how exactly do you want me to teach you mercy?" Sans rolled his eyes at me, "That's your job." You were a great help idiot. I leaned on the side of the couch. "So how exactly are you gonna explain this to Papyrus? He is the captain the Royal guard." Sans paused looking uneasy, he then snapped his fingers like he had figured it out, "I'll just use our deal. I'll tell him your my human, and that's the end of it." I looked unamused, "Well good luck with that."
The sun was beginning to set (don't ask me how I'm guessing the same way it snows down there), "So um Sans?" "What?" He growled, I guess down here you get used to being mean, "Where am I gonna stay?" Sans laughed, "In my room of course I have a spare mattress under my bed you can use it." The door was opened, "Hello brother, I'm home." Came a higher pitched growl. Papyrus froze when he saw me, "IS THAT A HUMAN!" He yelled, Sans walked in front of me. "Back off bro, she's mine." Papyrus looked unhappy about this, "So, what you capture a human and say it's your's and that's it!" Papyrus was looking pissed at this point. Sans shoved his hands in his pockets, "Yep." Papyrus stormed off into a room on the second floor, that I assumed was his. "Well that was easy." I looked at Sans confused, "What did you expect?" Sans laughed, "I expected to have to fight him, but I guess not, so whatever." Sans began walking towards the stairs, "Follow if ya don't want to die." I followed, reluctantly. He walked passed the room where Papyrus and went. I half expected a bone to fly through the wall spearing me, but it didn't happen. Sans walked to the last door opening it, "Welcome to your knew home, y/n."  The room was a mess, there was a self-sustaining tornado in one corner of the room. Sans pulled a mattress out from under the bed. He hopped onto his bed. "It's the best I got I'll work on getting ya something better to sleep on, since you haven't tried to kill me yet." I gave him a confused glance, "Why would I kill you? You haven't done anything to hurt me."  Sans considered this, "Well you have a point." I hopped onto the mattress it was surprisingly soft. "Well night the y/n." "Night Sans."
I woke up before Sans did, so I lay there thinking of my options, I could kill Sans right now and be done with it, I can wait and betray him, or I can fight him in a fair fight. I didn't really want to kill Sans, so I scraped these ideas. I could teach him to spare. It seemed like my best option. But doing that would risk my life, I would have to fight him to teach him. I'm pretty good at dodging attacks from a single person. "Whatchu doin?" Sans startled me, "Trying to figure out how I'm going to teach you to spare." Sans had been watching me. "You could start by teaching me feelings, cause ya know, everyone thinks feelings are weak. You have them I have seen you sad over something another." I looked at Sans he was lying on his back looking up. "What do you mean?" "Like sadness and stuff, I taught myself not to feel that." That's depressing, "Well you need to feel sympathy and empathy." Sans looked at me, "What do either of those mean?" This is going to be harder than I thought.