Fellswap!Silver

Check the Pinned post for TWs and Info. A take on the Undertale AU Swapfell/Fellswap. Creator is @moonyartandwriting.

[fellswapsilverundertaleau]

Fellswap!Silver, a take on Swapfell/Fellswap as well as Underfell and Underswap

This pinned post is a WIP and will continue to update in the future :)

WARNING! THIS BLOG CONTAINS CONTENT THAT COULD BE TRIGGERING, THIS INCLUDES:

Alcohol/substance abuse

Unhealthy coping mechanisms

Suicidal ideation and self-harm

Mental health troubles (social anxiety, depression, agoraphobia, savior complex)

Stalking and obsessive behavior within romantic relationships

Child death

Abusive parental behavior

Medical trauma/abuse

Body and Medical horror

If any of this makes you uncomfortable this blog might not be for you, feel free to block me or ask me about the specifics to confirm details.

Information under the cut (due to the fact that this will be a long pinned post)

Welcome to the official Fellswap!Silver blog! My main blog is @moonyartandwriting and I created this AU (or rather this specific interpretation). This AU was created back in 2020 but I only started posting about it in March 2026.

What originally started as me drawing Sans and Papyrus with Silver accessories turned into a full fledged AU. Combining my favorite assets from Underswap, Underfell, and all the various Swapfell/Fellswap takes, then turning it into my own thing.

This AU is full of my own head canons and interpretations of how Undertale's world and lore work as well as UTMV mechanics.

In character asks as well as requests are open! Though I am very slow so do not expect those in a timely manner XD (Regular asks and requests are also open)

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An Underswap and an Underfell AU collided into each other, causing their codes mixing to the point of no return. A full system reset was preformed in order to prevent the AU from collapsing on itself and as a result the AU was changed permanently. (Think of this AU more as a mix of UF and US rather than a more traditional Swapfell/Fellswap AU)

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The ones without imbedded links have content planned for them

History (Written)

Ruins (Concept art)

Snowdin (Concept art)

Waterfall (Concept art)

Hotland (Concept art)

CORE (Concept art)

Capital (concept art)

Castle (Concept art)

True Lab (Concept art)

Chara (Ref)

Temmie (Ref)

Asgore (Ref)

Glad Dummy (Ref)

Mettablook (Ref)

Papyrus (Ref)

Sans (Ref)

Grillby (Ref)

MK (Ref)

Undyne (Ref)

Worn Dummy/BooHoo (Ref)

Napstaton (Ref)

Alphys (Ref)

Muffet (Ref)

Toriel (Ref)

Frisk (Ref)

Asriel (Ref)

Gaster (Ref)

[moonyartandwriting]

I’ve been thinking about FS!S for the past couple of days sooooo here’s some colored doodles heheheheheeee 👀💕

(IVE BEEN MAINLY THINKING ABOUT DARE SHES BEAUTIFUL AND WONDERFUL ARGHHHHHH I gotta make some fully rendered art of her when I get the chance 🥺💖🐟)

OH MY GOODNESS

i have been lowkey staring at this in my inbox all day since i woke up, your colors are gorgeous as always

i LOOOVEE how you draw Dare, her hair especially

you know me so well! i am always thinking about papyrus....

Shades and Rus is so great, his facade stays in place for about 5 mins before he joins in the insufferableness (/pos)

[fellswapsilverundertaleau]

Im making a banner for this blog that has objects that represent the main cast, these are the ones i have so far.

I’ll talk about them more when I’m done but here’s the sketch for Chara, Temmie, and Asgore.

I need to do more fox studies bc Chara’s mask doesn’t look right.

[fellswapsilverundertaleau]

(Brief) History of the Underground

first part originally posted on @moonyartandwriting

After the death of Asriel and Frisk there was an argument between The Underground’s monarchs.

Toriel believed that there was still good in humanity, Frisk was evidence of that. Asgore believed that humans should pay for all that they’ve done to monsterkind and wished to launch another war.

The argument was loud and destructive, witnessed by several of the palace’s staff. Eventually Toriel won by pointing out the hypocrisy within Asgore’s argument, claiming that he was just as bad as the humans they feared. Asgore, enraged and ashamed, fled from the palace and into The Ruins and hasn’t been heard from since.

Toriel tasked The Royal Guard with keeping peace throughout the Underground and to capture any humans that make their way through, by any means necessary.

Eventually a few humans fall, and are taken in to the queen. None make it to the surface.

In an effort to heal her grief, Toriel had lied to each human, saying that there was no way through the barrier because it was designed that way. She kept every human under her care, until eventually they perished.

Humans can’t survive solely off of magical food like monsters, they need vitamins and nutrients (which monster food mostly lacks). Thanks to the help of the Royal Scientist after analyzing the human corpses, Toriel figured this out.

She continued to keep this a secret from the rest of the public, swearing the scientist to secrecy and forbidding any other monster to research human biology. There are constant weekly raids on the dump and check-ins on houses to ensure no information about humans gets out. Toriel feared that if monsters knew how weak the human body could be that it could mean the end of ensuring humans safe travel to her castle.

Toriel began giving the humans a choice, she would either kill them instantly, or she would allow them to live for a little while longer before the burden of living became too much for them.

The Capital has almost stayed entirely unchanged for years, meaning it is severely underdeveloped technologically compared to the rest of the underground. This isn’t true for the rest of the Underground however.

~~~

Waterfall is the most highly developed area, it is the home of Royal Scientist Undyne after all. It utilizes the various sections of flowing water in order to generate electricity. It's hard to find an area of Waterfall that isn't swarming with Undyne's inventions, what used to be the most tranquil place Underground is now a cybernetic hellscape. Hidden (and not so hidden) cameras are set up anywhere Undyne could reach so she can monitor who passes through the area. Many of her abandoned projects still roam around, causing chaos wherever they go. Most monsters have cybernetic enhancements given to them, under the threat that if they don't accept they'll disappear without a trace to become one of her test subjects. The few untouched places are the wishing stone room, the garbage dump, and the shrine room (at the request of Toriel).

Hotland is the second most, though much less so than waterfall (since Undyne hates traveling there for work, she wishes to stay in Waterfall as much as possible, therefore it hasn't received the same overall quality upgrades as Waterfall has). The CORE is still its pride and joy, it being the peak of monster engineering, providing steady magic generation throughout the entire underground. It's a hot spot for geothermal engineering. Puzzles use lasers and steam, which is also used as transportation. Most of the other scientists live here in order to be closer to their work, and to the CORE in case it needs repairs.

Snowdin ranks third with the help from the locals (and favors called in to the royal scientist). Though it's nothing to be compared to Waterfall or Hotland, it's still far more technologically advanced than the Ruins or Capital. Wooden contraptions are built alongside slippery ice to facilitate more complex puzzles. Most of the more advanced traps are built by Papyrus (who enjoys all types of metal work) or Undyne when she visits. Lights and running water are only within the town's limits, only idiots go into the forest after dark (even if you go in during the day you are considered foolish). Besides, if non-locals want to warm up they can either go to Grillby's or purchase a room at the inn.

The Capital is nearly medieval in the scientific field. Outside of a select few technological advancements that improve the quality of life, it's been unchanged for centuries. Streetlights were put in place to illuminate the streets, pipes were installed for running water, other than that there isn't much else that is permitted. The only means of transport is by foot and the puzzles resemble ones from the Ruins.

The Ruins are the least developed region. Due to the few monsters that live there, not much has changed since they had first arrived in the Underground. The ancient puzzles are the only semblance of anything that could qualify as technology left in the place, and even then they are rusted over and barely function. Plants have taken over the place (courtesy of the former king) and the monsters have adapted well.

[fellswapsilverundertaleau]

Fellswap!Silver Concept art

originally posted on @moonyartandwriting

[fellswapsilverundertaleau]

Fellswap!Silver Doodle dump

originally posted on @moonyartandwriting across several posts

[moonyartandwriting]

BOO YURI JUMPSCARE!!!!!!!!!!!!!!!!!!!

AHHH ITS THEM ITS MY GIRLS TYSM‼️‼️‼️

I LOVE THEM IN YOUR STYLE 😍😍

[moonyartandwriting]

More about Toriel (Fellswap!Silver)

Toriel (Buru) sees all her subjects as "her children" in a way. Not as literal as how she views the humans that fall (who she views as people she is entirely responsible for and who are dependent on her), but more in a teacher sense (i have no other word for it atm).

(more ramblings and details under the cut)

She cares deeply about their struggles, their lives, their jobs, their relationships, their pretty much everything. When she gets the chance to talk to someone willing (key word willing) to share they will spend hours going on about everything. However she knows her limitations as an individual and as a queen, she cannot spend every waking moment tending to the innermost aspects of her subjects lives. Instead she often just allows herself to just be heavily involved in her royal duties. Attending every gala, fundraiser, event, showcase, holiday, festival, everything that is a wider spread social event that she could feasibly go to.

People have learned not to share information unless absolutely necessary with their queen directly. They know she will get her claws involved in everyway she can and it tends to not end very cleanly for any party. This is especially so if a human had fallen "recently" (recently being a time frame from around 2-8 years), Toriel's emotional reasoning and mental stability is very fragile during this time. Everyone is aware of this fact including Toriel, who goes along with it because she is at least in the right mind to know its for her subject's safety.

This has led to an informal and unspoken agreement between the royal guard and Toriel. The royal guard purposefully omits information from written reports that could potentially cause any unwanted interference from her highness. The highest members of the royal guard (Sans, Alphys, and a few others) will relay these written reports to the queen, using specific and highly engineered language to ensure they do not set her off (she can tell if you're lying by listening and feeling someone's signals sent off of their body and soul unconsioulsy). Toriel knows these reports omit information and sometimes blatantly lie, but as long as the reporter words things as instructed then she will not interfere. She knows that it's a necessary evil since if she did interfere every time nothing would get done, however mistakes are punishable.

Sans is the de facto giver of information. He gets along with the queen best personality wise, he's great at keeping secrets and policing his language, he's almost always on the job and looking for an excuse to work, he's one of the best underground at controlling his magic, and selfishly he also cares about Toriel. He wants to know that she's doing well and will occasionally throw her a bone (*wink wonk*) by giving her extra information or outright telling her things he thinks she should hear. This is only done when he truly thinks that she needs to get involved, either to set an example or because it involves more personal aspects that the guard isn't necessarily equipped to handle.

[moonyartandwriting]

Fellswap!Silver alphyne's dynamic (undergroud at least) is basically Red Flags by Tom Cardy feat. Montaigne if ykyk

[moonyartandwriting]

someone HAS to kill these guys……

[moonyartandwriting]

Dare doodles for the people, im filling empty space in old sketchbooks and wanted to practice drawing her more.

[moonyartandwriting]

idea where chara wears the faded ribbin more like a tie than an actual ribbon…..

[moonyartandwriting]

omg i finally finished Shades’ ref

this is my boy, my son, my precious

(lowkey embarrassed abt how much i like it)

[moonyartandwriting]

About Sans and his addiction (Fellswap Silver AU)

TW FOR SUBSTANCE ABUSE AND MENTAL HEALTH PROBLEMS

I've talked about how Sans struggles with addiction and how he wouldn't recover until after surfacing. I think what would get him to decide to work towards it would be the fucked up realization that

If he had a choice, he would choose alcohol over Papyrus.

And that's really bad. That's his own damn brother, the only person that made the Underground somewhat livable. He's the reason why Sans has worked towards anything and why he's still alive. That's the only family he has left, they're the only two skeletons alive and the Underground was way too dangerous to consider literally anyone else family. Family was all you could trust Underground and if Sans doesn't have that he really has nothing other than his own vices.

I don't think Papyrus would force this revelation (because he has a LONG LIST of problems regarding his relationship with Sans and his own mental health issues), but I think that someone else would. Most likely Toriel (since she's third closest to Sans after Papyrus and Alphys) due to her opinions on family relationships. Alphys is another contender because she would be one of the few people who knows how bad he struggles with addiction and how much Papyrus cares for him. Even Chara could be the one too do it, which would be a serious reality check. TBH all of them are contenders and perhaps it could be a group intervention.

I think that the process would be (VERY LOSELY) something like

the intervention/confrontation happens

Sans and Papyrus end up living separately for a little bit while Sans starts recovering, both at their therapist's and friend's suggestion

they both get further along in the healing process

they move back in together much healthier and happy people :) (because i can't write sad endings for my characters ever!!!!)

Anyways lemme know what other stuff i could add or if i got anything wrong/used the wrong terminology. I don't struggle with addiction firsthand however do have family members that do, so let me know if anything i said was inaccurate or offensive.

If you have any questions or wanna know abt this AU don't be scared to shoot me an ask!!!!!!!!!!!!!! :DD

[moonyartandwriting]

ive been thinking abt FellSwap Silver Alphyne a lot the past few days……

(do not talk to me abt the hands, i was in a pit of suffering that i myself created)

more abt them under the cut....

Quick warning for mentions of stalking behavior!!!!!

I mentioned in my first FellSwap Silver post that Undyne falls for Alphys first but didn't really go into more detail abt them.

I would say that they don't ever get together underground for two reasons.

Undyne is kind of crazy, and she becomes very stalkery and yandere-esque. The cameras set up throughout the underground were set up as her way to catch glimpses of Alphys since neither of them get out often (Alphys only leaves for work and Undyne is mostly stuck in her lab experimenting), she did this under the guise of wanting to keep a look out for any humans that might fall. I imagine that Undyne is regretfully very busy with her experiments and doesn't have much time to plan out scenarios for them to meet formally. Her time outside of experimenting is also very restricted because of her massive workload.

Alphys' horrible agoraphobia/paranoia/anxiety makes it so that she's barely goes outside and interacts with new people, she has maybe met Undyne once or twice at work meetings but other than that she doesn't know her at all. So there would be no way that she seeks out Undyne's company.

When they do get together it's after surfacing and LOTS of therapy on both ends. Undyne learns to chill tf out and reign in her behaviors/emotions. Alphys has learned exercises for dealing with her fears and is on medication to help her mentally.

I feel like they would both meet at an anime event or at a store. Undyne mentally panics because she still has a crush on Alphys but is worried about how she might see her after how she acted underground. Alphys is the one to approach Undyne since she knows they both have a common interest and also because her goal post-surfacing is to get to know more people.

[moonyartandwriting]

Doodle dump! These are from my own version of SwapFell/FellSwap. All are just of the two skeleton brothers, who are named Shades (Papyrus) and Tempest (Sans).

Most of these are old but i wanted to share them anyways for people to get more of an idea of how they act.

My favorites are the collage since it has references to their personalities and likes, and the second one because it makes me laugh every time i look at it.

[moonyartandwriting]

Welp, guess i better rip off this band-aid. I have my own version of SwapFell/FellSwap, i've been calling it FellSwap Silver (due to Sans and Papyrus having false silver teeth/bones/accessories as opposed to the typical gold).

i thought a lot on how i want to structure the AUs (for another separate project ill post eventually) and was thinking that this AU would be a result of a Swap and Fell's universe colliding and combining the code of both into a singular one. AKA me taking my favorite attributes of the different AUs and adding my own things here and there.

This is Shades (Papyrus), Tempest (Sans), Rex (Alphys), Dare (Undyne), Charr (Chara), and Spooky (Napstaton). These are all quite old art so just know that these designs are kind of out of date. These are also the only ones i found art for in my backlog, so no Toriel, Asgore, Gaster, or anyone else atm (sorry).

Lets get to know these freaks (/pos)!!! TW for mentions of substance abuse

Sans - He/him, 6ft. Personality wise he's actually pretty similar to Underfell and Fellswap Gold Sans (manipulative, controlling, pessimistic, and rude), however has both the energy, strength and temper of Underswap and Swapfell Red Sans. Pretty much always reeks of cigarette smoke, ozone (related to his magic), and expensive liquor/cocktails. Family bread winner, given his former scientific status as well as high ranking in the royal guard. Constantly indulging in his vices (drinking, smoking, gambling) when not working, could be considered a high functioning alcoholic but is very sensitive/defensive when people bring up his reliance on it. His brother is the only person who can drag him away from his work. He loves to pretend he's the most responsible person he knows, while in reality Papyrus is the one making sure he's keeping his shit together. When he's not working or out drinking, he's at home falling asleep to NTT reruns with an unlit cig in his mouth and surrounded by savory junk food. When he's banned from working or drinking, he'll head to Alphy's house to keep her company and to watch whatever she has on (he usually ends up falling asleep and Papyrus has to pick him up). Post-surfacing he ends up consuming a lot of trashy/cheesy sci-fi and mystery shows. His favorite food is anything that could fall under the category of savory and salty gas station/road trip food (combos, jerky, chex mix, etc). Missing both a tooth and a few digits from his fingers (covered by gloves 24/7), both were replaced with silver replicas. His magic takes the form of lightning/electricity alongside the bones and blasters.

Papyrus - Any, 7ft. His personality and aesthetics takes after Underswap and Swapfell Red Papyrus, while his aspirations and interests align more with Fellswap Gold Papyrus. Keeps fresh herbs for cooking, gardens as a hobby (courtesy of Asgore). Their mission is to get their friends and family to fucking chill for once (it's a full time job), spends a lot of time thinking of ways to get Sans to stop drinking/working and for Undyne to dial down her experiments. He rarely takes his job as a member of the guard seriously, only really got it because of his brother, and is always finding a way to get out of work. She always needs some kind of fidget/puzzle/trinket on her person. They only know how to cook to get their brother to eat something healthy, otherwise they'd only ever eat sweets. While she's usually very detail oriented, she does occasionally leave stuff around the house and it drives Sans up the wall. Insomniac, has trouble sleeping a night leading to him taking a lot of short naps during the day and early evening. He listens to a lot of Tool after the ascension. They have two silver canines and silver mandible supports. They utilize layer clipping and fucking with physics way more, though still uses bones from time to time.

Chara - They/them, 4'6". Stayed in the underground way longer than most humans, leading to them having a very ragged/unruly appearance. Can hold their own in a fight due to them getting in trouble with bullies on the surface. Very snarky and sarcastic, loves bantering with Sans and Papyrus. Impatient and gets bored easily, which leads them getting into a lot of trouble usually. Doesn't really care about their appearance, and will wear the same pair of clothes multiple days in a row. While still a chocolate fiend, they also really like sour candy and fruit.

Alphys - She/her, 6'2". Has really bad agoraphobia, social anxiety, and paranoia, causing her to rarely leave the house unless it's under guard related duties. She really likes high fantasy and isekai anime, it gives her a sense of comfort and escapisim. The only people she really interacts with is Sans and Toriel. Whenever she does go out it's always with other guards at her side. Apart from her room, her house is basically a huge training dojo, she's almost always sparring to keep her strength up (given that her magic isn't all that strong, she focuses on brute force and intent). The only time she's comfortable making decisions is when they're quick, in the moment, and on her feet. Favorite food is instant/package udon with two runny eggs. She becomes an EMT on after surfacing and going through a great deal of therapy.

Undyne - She/her, 7'5". Has a nasty cruel streak and easy to piss off, get on her bad side and it's over for you. Gets distracted easily, prisoners have escaped by catching her monologuing many times. She really likes extreme/splatterpunk horror, especially of the body horror variety. The amalgamates are on the less extreme level of her experiments. She is very strong magically, making her a force to be reckoned with, she uses it to her advantage by intimidating monsters into doing what she wants. She really likes seafood of any kind, though is partial to squid and octopus. Falls for Alphys first after seeing her at a royal conference, Alphys got one of her references to a niche anime and has been fascinated with her ever since. Becomes way more chill after getting to the surface, dedicates her life to launching a biological research center focused on giving voices to monsters and minorities withing the scientific field.

Napstaton - They/them, 7'7". Most of their acts were influenced by the media they watched with Undyne when they hung out a lot, though did omit some of the more gruesome details (though that doesn't mean they aren't violent). Still visits Mettablook, he never really responds to any sort of questions they ask and answers in very short sentences. The one of the very few who kept up hope, often broadcasted updates on the human soul situation and urged people to keep a look out as well as air human trivia segments (with very inaccurate information).

Feel free to send me an ask if you wanna know more about them or this AU! i can't promise to answer right away but i'll absolutely try to get to it :D