Till Death Do Us Part - Dust Sans x Reader

Surprisingly, you had a rather restful sleep. That was until you felt someone shaking you rather aggressively. Opening an eye, you merely sigh as you see Dust impatiently shaking you about. Upon noticing you're up, he casually walks over to the lounge area of the room, sitting down on the sofa and putting the TV on.
"Goodmorning to you too.." you grumble, glaring at him slightly.
You were never a morning person, which made you quite irritable before you've woken up properly. But being woken up randomly whilst you were still trying to sleep? That was something you hated a lot, so it was hard not to feel annoyed. You quietly get up and head into the bathroom, looking at yourself in the mirror. Your hair was tangled and you had a few scrapes on your body from traversing the underground, but other then that, you looked pretty much alright. You were about to leave when,  echoing around you from seemingly nowhere, was a small voice that seemed to belong to a little girl.
"You look really pretty, miss. Such a shame you have to be involved with such loathsome creatures."
You couldn't see anything anymore as the room got incredibly dark; only the mirror being still visible. You could see your reflection and a pare of red glowing eyes behind you around the same height as a child.
"W-who are you? What d-do you want from me?"
The girl seems to chuckle.
"I don't want much. Just for you to gain enough LOVE and EXP to bring me back to reality. You see, I'm in a void right now. And so I can't interact much with the people I want to. But if you get enough EXP, you'll get lots of LOVE, which will give you enough DETERMINATION to make me at least corporeal so I can do the things I need to."
You pull a perplexed expression. What was this kid on about? Like you would ever agree to do that.
"And why would I ever even consider doing that?"
"Because, I can take you back home."
You pause, contemplating this. For all you knew, she could be lying, but at the same time, you couldn't prove that she was. And Nightmare had no idea how you even got here, so him knowing where your home may be is slim to none. So, you begrudgingly agree to help.
"Fine, but can I at least know your name?"
"My name is Chara. No need to tell me yours, I already know who you are. Oh, and one last thing....~"
You look behind you to see a young girl with short brown hair in a bob. She wore a green and yellow striped jumper and brown shorts with black tights underneath. She seemed to also be wearing some brown ankle boots. You noticed she was extremly pale, making her rosy cheeks very obvious. But the creepiest part about her were her eyes and smile. Her eyes held a horrible insanity behind them and appeared to glow a soft red colour. In her right hand was a red knife. She was extremely creepy, and you could only assume that this was who Chara was.
"Don't even think about telling anyone about our little encounter. Otherwise, you'll regret it Y/N."
She began laughing, black ink leaking from her eyes as her face began morphing. It was awful and you couldn't help but scream in fear as her laugh echoed all around you, getting louder and louder. You fell to your knees, clutching your head as you screamed, hearing nothing but her laugh. Until you were brought out if it, back in the bathroom, Dust frantically shaking you and calling your name.
"Y/N!! Can you hear me?!"
You wiped away the tears that were running down your face as you stare at the floor, in shock and unable to process what just happened. You had never felt so much fear ever. It was weird, because Dust seemed quite panicked.
"Hello?! Y/N?!"
"I-I'm here, I'm ok.." you just about whispered.
He sighs, moving a little aways from you to sit beside you. It only now occurred to you that you were both on the floor.
"Care to explain what the fuck just happened?"
He didn't seem annoyed or angry, more so a little concerned and unsure of how to feel. You knew what the risk of telling him was.
"I can't... I can't tell you.."
"What? Think I won't believe you?"
"No, it's not that. Just, I've been told not to tell you. That's all I can say.."
He stares at you with a questioning glare. It's obvious that Dust wanted to say more, but he knew that there wasn't much that could be said now if you weren't allowed to talk about it. He slowly gets to his feet, gingerly holding his hand out to you. You gratefully take it and pull yourself up, awkwardly looking at each other in the small bathroom.
"Ahem, anyway. Nightmare wants to see you whenever you're available. He mentioned it to me before you started screaming and being weird in the bathroom."
You nod, walking back into the bedroom. You figured there was nothing you actually needed to really do, so you exit the room, taking a deep breath. You began mindlessly walking the halls of the castle, taking note of the decorations such as statues of monsters and paintings of places you had only ever seen in your dreams. You noticed that in each painting, there was a huge tree covered in apples. Some black, some golden. Was the tree important somehow? You shake your head a little, noticing a large black door at the end of the hallway you were currently walking down. Small, gooping tentacles seemed to be coming out of the cracks of the door, which made you take a small guess that it was Nightmare's. Sighing, you try to push away any and all thoughts begging you to turn back, to run away from the monstrous skeleton. To hide away from the literal nightmare on the other side of the door. Ignoring everything that begged you to leave, you twist the knob and push the door open.
"Ah.. Y/N... I didn't think you would come. But, now that you're here, I can get started on my research. It's always nice to understand why things happen so I can keep it in mind in case another... scenario..  like this is to ever happen."
You manage to tear your eyes away from his as you inspect the room, closing the door behind you. There were no windows, and it seemed more like an office than an actual bedroom. Although, there was another door at the back of the room, so maybe that led to his bedroom? There was a desk that Nightmare was currently sitting at in the middle of the room which a chair on the other side of it for, you could only assume, whoever had to come here to talk to him. There were a few bookshelves around the room, but over all, the place looked like what you would expect a gothic office to be like. Hesitantly taking a step forward, you felt fear bubbling inside of you making your knees feel weak and your hands tremble ever so slightly. Your throat felt tight, and it was almost impossible to breathe properly.
"Do not be afraid, Y/N. If I wanted to hurt you, I would have already. Now, take a seat. We don't have all day."
You nod a little, moving towards the desk and sitting down on the chair, subconsciously tapping your fingers against the hard wood of the desk to try and soothe yourself.
"What do you want from me?"
"Nothing major. I'm just going to ask a few questions, that's all. So, you say that you 'fell from your bedroom window' and 'a black portal opened up and sent you into the underground', correct?"
"Correct."
"I've been looking through files of each Timeline and Alternate Universe for anything about a human falling from the surface through a black hole straight into the underground. But, as you could imagine, I've found nothing of the sort."
"What does that mean then?"
"In short terms, it means you are an anomaly. You aren't exactly supposed to exist, yet you do. Clearly you exist for a reason, but all I can tell you is that you're special. Take that information as you will. Now, I hope this isn't too embarrassing for you, but I wish to see your soul. Can you get it out for me?"
You sit, quite baffled by the request. Take out your soul? How would you even do that? The only way you knew it was possible to get your soul out was when Dust forced it out and when you enter a battle with another monster in the actual game. But to actively make it come out for no reason? You couldn't even begin to imagine how something like that could be done.
"I.. I don't know how to do that Nightmare." You admitted quietly, earning a strange look from him.
"You don't know how to take out your soul? Interesting. You really do know nothing. It must he nice having such blissful ignorance. Just hold your hand to your chest and grab the air. Then, once you're sure you feel like you're holding something, pull it out."
With all honesty, even though Nightmare explained what to do and even demonstrated it to you, you still had no idea what was going on or how to do it. However, you had a sneaking suspicion that Nightmare wouldn't be best pleased with you continuously telling him you had no idea how to do it, so you just tried your best. Holding your hand to your chest, you awkwardly grab the air, giving Nightmare a funny look as, like you would expect, it was just air and you felt nothing. However, after a while, your hand felt heavy and quite warm, almost like you were holding something that wasn't really there. Was this what Nightmare was talking about? Wasting no time, you pulled, your soul appearing in front of you. Like earlier, it was grey and translucent, giving off a feeble glow whilst it hovered awkwardly in front of you. You could see it pulsating slightly.
"I-I did it!" You exclaimed gleefully.
But Nightmare didn't seem best amused. In fact, he seemed incredibly startled, and hesitantly reached out a hand to touch the soul, only to phase right through it.
"Is, uh, is that supposed to happen?"
"No? Any soul can be touched and messed with. There's no exceptions. The colour of the soul is something I expected to be a different colour, but..... not holding a physical form? Seeing through it? That's something I never thought could be possible." 
Both of you sit in silence. And despite having a vulnerable part of yourself out in the open, there was a strangely comfortable vibe between you. Maybe Nightmare wasn't so bad? 
"There's not much else I can ask of you because you clearly know just as much as I do: nothing. So, instead, over the next few days, I'm going to start running physical tests on you and your soul." 
This caused you to flinch slightly. Physical tests? Surely that would hurt. And you didn't fancy the idea of becoming some lab rat for Nightmare so that he knows a tad bit more about the universe he's lived in forever. Fortunately, Nightmare seemed to notice your growing discomfort.
"Don't worry about it Y/N. None of them will hurt. And after I'm done, I'll try and find you a way back to wherever you came from. Is it a deal?"
You thought long and hard about it. Chara had already offered to take you home, but unlike Chara, you wouldn't have to be killing anyone. On one hand, you wanted to decline, in fear of what Chara might say and do, but on the hand, this was an opportunity you weren't willing to give up because of some ghost, so....
"Alright, it's a deal."
"Wonderful.~ You may be dismissed. It's still time for breakfast, so go and grab something to eat before I hand out everyones assignments. I'm sure you want our group to at least like you a little while you stay here, so try to make some friends."
"Will I be given an assignment too?"
Nightmare chuckles softly, a small smile gracing his features. For someone so creepy and evil, he sure could be rather nice.
"No, I'm not sending you to certain doom. These assignments consist of things that someone such as yourself won't be capable of just yet. Maybe in the future I can let you assist them, but as of now, you clean the castle and do basic household chores while you stay here."
You expected to be treated worse than this considering you were a pet, but so far you've been treated just like everyone else. Were they really as bad as they presented themselves to be? You nod slightly to show you acknowledged his words and stood up.
"Alright, thank you."
Nightmare merely nods and turns his attention towards some paperwork on his desk. You left the room, closing the door softly as to not disturb him, and made your way to the dining room. It really wasn't all that bad here aside from a few skeletons who weren't very fond of you. And all you had to do was clean the place a bit? You weren't going to lie, this wasn't a bad deal. But, you were more excited to finally return home to your friends and family. And hopefully, not too much time has passed whilst you're staying here. Turning the corner, you walk into the dining room. Cross and Dust were the only ones still at the table eating, and you could hear Horror in the kitchen fumbling with things. Looking at the table, your attention was drawn to a plate full of pancakes in your spot, and you wasted zero time sitting down and getting started on eating. Dust sort of glanced at you occasionally while he ate his own pancakes. It was silent across the table, but it wasn't awkward. It was more like a nice family moment than anything, and the thought of that brought a small smile to your face.
"What're you smiling for, hm?"
You look up at Dust who was studying your features.
"Huh? Oh, nothing, I'm just thinking of stuff, that's all."
He lets out a reassuring hum in response and carries on eating. Horror wanders into the room with his own plate full of food and you couldn't help but almost choke as you saw that his tower of pancakes was as tall as he was. But, he seemed happy as he began eating with his mouth like a dog, getting mess literally everywhere.
"Anyway-" Dust manages to tear his eyes away from the scene, "-what did you talk about with Nightmare?"
"Well, he asked me general questions like how I got here and stuff, and eventually he asked to see my soul, so I showed him. But uh, turns out it's not physical? So you can sort of just phase your hand through it??"
Dust blinks, not entirely sure how to process what you just said.
"Phase through it?"
"Mhm. And he also wants to run some physical tests on me."
He pauses eating, now a little more serious as he properly turns to face you. A little startled, you do the same.
"Physical tests? I don't like the idea of that."
"Why do you suddenly care?"
"I don't. But I don't agree with the whole 'testing on people' stuff. I just don't think it's practical."
You merely roll your eyes, finishing off the last pancake and wiping your mouth with the napkin that had been provided for you. You didn't think it was too big of a deal since you weren't going to be in pain at all and you could go home after it.
"It's not really that big of an issue that you're making it out to be Dust. Besides, he'll help me find my way home after it, which is a deal I'll take any day."
Dust doesn't really respond, but he picks up both your plates and heads to the kitchen to clean it. Not saying anything yourself, you head back to your room considering you didn't need to stick around to find out what missions the others would be given. So, opening the door, your flop onto your bed, only to hear a very familiar voice.

"So, let's talk about what was said between you and Nightmare, friend."