A/n: So, after reading all these amazing fan fictions out there, I thought why not write one of my own! So here I am! I am new to this fandom, so please be kind. Any sort of constructive criticism will be accepted with open arms so do not hesitate to go ahead and provide some feedback. I would love to hear from all of you!

Before I begin, I would like to clear a few things for my readers.

First, this story may get dark at times and may deal with future major character deaths. So if you are not interested or simply do not like to read stuff like that, please skip.

Second, in my headcannon, Frisk is a female and all the pronouns used for them would be feminine.

Third, this story will and does contain SansxFrisk or other ships but will not be overly shippy or fluffy. I am aiming to develop deep relations between all the characters (romantic or aromantic). In addition, the story will majorly revolve around the skele-family and a certain Fell flower.

Fourth, all the names in italics will refer to the fell universe characters (which may usually be prepositioned with a 'Fell' or an 'Alter'.

Fifth, ratings may go up later. May being the keyword.

That is all for now, hopefully! Enjoy and let me know what you think! Will hopefully be updated every Sunday.

Prologue

'Embrace it…my child…'

His sternum burned. Pangs of heat mixed with tinges of nervousness shot at his bones, beads of sweat trickled down his ivory cervical and wet his fur-lined jacket. The affection had gnawed through his vertebra and now, slowly phased down to his femurs.

'You should not run…'

"n-no…you cannot…" He ran, gripping his jacket with sweaty phalanges and his mind with dialectics.

His SOUL pounded in his chest like a strong beat on a dilapidated drum as he ran through the endless Void, trying to escape the testimony of his actions. He had berated himself, repeatedly for bringing in what was lost; after all Space and Time was not meant to be pried into, let alone exploited.

The shadow loomed over the bottomless pits, cluttered with various portals to places he had not the fortune to explore. Not like he wanted to at the moment.

'This…this manifestation is real, my son.'

The shadow boomed.

'You saved me, son…you saved me…'

He kept running, his fibulas threatening to tear apart if he maintained his pace.

'You did not forget.'

"gaster!"

Sans shot up from the hard metallic desk, sweat pooled beneath him; leaving a huge patch of wetness on the white sheets of nights of work. He would have cursed but he was too busy coping with the recent nightmare.

His breathing was riddled with fatigue and his eyes blank in an ominous foreboding.

"Yes?" The aforementioned spoke, his gentle voice and calmed stature somewhat comforted the short skeleton.

Gaster had turned around from his metallic working table –producing a menacing clink in the process, deeply distressing the other scientists at work- and had stared at his son blankly, not as if he had any emotions he could muster up with a face like his.

Not as if he was not handsome…he just lacked the…visage.

And the vitality for actually caring about the same.

Much like The Human, he would have pointed out to himself if he were not so occupied with the equations at hand. Or his sons deteriorating mental health.

" 'm fine." Sans replied curtly, wiping the sweat from his temples and subsequently cursing at the ink blotched papers on his desk that were wet and dripping from his sweat.

"Is it a-another nightmare?" Alphys asked, turning her head towards the skeleton behind her.

"nah. just a bad dream." Sans replied, pulling up his lips into his signature goofy grin, "guess i gotta put a lil' more BACKBONE into what i do." He laughed, apprehension slicing through his sloppy façade. He had always been so good with wrapping up his feeling in, so why could he not now? Had the RESETS picked on his mental health to such an extent that he had lost all control over his emotions? Or was it the fault of the situation they were in now?

He wanted to believe the latter.

Gaster frowned, looking back at his neatly scripted equations.

"The Anomaly has been leaving ripples in Time and Space…certainly; it is not the doings of a body…or bodies knowledged in the same." Gaster monologued, his voice flat and smooth, "But it baffles myself as to why they would merge the universes together…"

"To destroy them of course." Alphys crisped, scribbling down messy lines of mathematical variables and constantly pressing out creases in her red and black shirt. The maniacal scientist had joined them soon after she had found out that they could not fight the Anomaly or its army alone, they had to work together despite their…differences in opinion.

The Aberration had not quite come alone this time, were not quite human and certainly not quite friendly. They were shadows, blessed with the strength of at least one-and-a-quarter monster SOULs and certainly, there were a lot of them. However, their lack of fighting skill made up for their numbers.

They guarded the paths, exterminating any weak monsters in their way –they did not dare to touch the strong ones lest they lessen in numbers; and subsequently, in power-. After defeating the monsters, they would absorb their SOULs. Each SOUL absorbed meant more power for them to traverse through Time and Space. And more power meant more destruction.

The pure nature of the 'Anomaly' –as they had grown accustomed to call it…or them- was unknown. The only things they knew were that one very prosaic day a bright light flashed in the false sky of the Underground, following the emergence of the…Anomaly.

The Anomaly had risen from the grounds; had descended from the skies and had slain everything and anything that lay in their way; destroying houses, ripping away the snow-laden flora of Snowdin town and tearing through the dewed grass of the Waterfall. They had even terrorized monsters, drove them out of their abodes; killed them, even. Prince Asriel had posted all of the Royal Guard to take care of the little business; little had he known…this business was anything but little.

Sans had sensed the Anomaly long before its arrival, but with the lack of knowledge about the aberration, he could only blame Frisk or his daughter for the same.

Frisk and his daughter alike, refused to have anything to do with the deviations caused in Space and Time, leaving Sans devoid of any plausible explanation. Dismissing it to be the repercussion of his degrading mental health, he flashed a strained grin to his family and left for the Laboratory in the Hotlands.

He discussed the matter with Alphys and Gaster later. However, they could only provide so little about what was happening to him. After hours of deliberation, they could only come to one conclusion. The irregularity was caused by a RESET. Sans was reluctant to believe it at first, but finally gave up for the sake of himself and his mind.

"It has to be a RESET"

He had told himself. For the first time in life, he actually hoped that either Frisk or his daughter had lied to him.

However, in the wake of the circumstances he faced at its arrival, he was sure it was no doing of a simple RESET. Or even Chara.

It was something far worse.

And the horrific part had not even begun yet.

Almost a few hours past the Anomaly's arrival, another flash of light –albeit darker this time- dominated the sky. The earth beneath the monsters shook as if it were unloading the weights of a thousand years from its back before it lay to a final rest. The river currents in the Waterfall grew turbulent and fierce storm whipped up in the womb of the Underground- the CORE.

After the storm cleared up, hundreds of new monsters had shown up. Not completely new, per say but different. A different version of each monster that sheltered in the Underground had appeared. The new monsters were dressed either in blacks, reds or yellows and were very hostile and confused. Even the surroundings had been mutated in some places. The flurry of green trees just outside Snowdin had been replaced with dying singed ones. The famous 'Grillby's' sign had been replaced by a cooler –quite literally so- 'Chillby's' sign.

It did not even take a moment for the former Royal Scientist and his son to decipher the happenings.

Two universes had collapsed into one another.

And they exactly knew which ones.

The Underfell universe residents were baffled, and they had all the right. The time lag had drove many of them to insane killing sprees, many to irritable moods and yet more to depression. The Undertale residents were equally confused and scared of getting hurt by their seemingly savage and heartless counterparts. In the same fear, they started fighting their counterparts, forgetting about the real enemy. The Anomaly.

Sans was sure it was their doings. No other monster or human -than a select few he knew- could hold such immense power, let alone be the cause of collapsing universes. A massive amount of energy and SOULs were required to make universes collapse and monitoring the Anomaly's behavior, he could positively say that they had a hand in it.

The situations had gotten worse in the Underground. The snow was covered with soot, the air with smoke and the atmosphere smoldered with the scent of death.

Sans and the others decided it was time for them to meet their counterparts and ask them to join forces or else more monsters would die and more destruction would be done. Frisk, as always, was the ambassador of their Underground and led them to her Fell counterpart.

The other Frisk, Undyne and Flowey (nobody wanted the yellow flower there to add to their misery; Sans in particular. He had only stopped Gaster Blasting the flower to oblivion when Frisk assuredhim of the flower's innocence and had tucked him away, behind her)were more than accepting of their offer but the others…were not very trusting of the proposing party. Especially Fell Sans, Toriel and Papyrus. They had refused to help them at first –willing to 'burn 'em up' up even- but eventually gave in, they knew they had to save themselves and the ones close to them, even if it opposed their natural instinct or hurt their vanity.

Pride or not, survival came first.

The former queens of both the universes accompanied by Prince Asriel, sated the masses and asked them to move to the CORE, where they would be guarded and taken care of until the situations got in hand. Fatigued and hurt, the monsters complied. However, before they could act on the decision. The Anomaly acted up. They attacked, disrupting and scattering the crowds to places beyond the immediate grip of the group. They were only a handful, but the fear that they had etched into the frail monster minds was enough for them to scare the monsters out of their wits.

The group could barely gather a few people before retreating, only Sans, Papyrus, Undyne and their counterparts stayed back to fight away the rest of the Anomaly.

Since that day, the group divided in four sub-groups.

Fell Sans, Undyneand Fell Papyrus were on guard on the paths, dueling any Anomaly in the way. While Papyrus, Frisk, Fell Undyne and Fell Frisk followed, healing and tending to any hurt monsters that had been lucky enough to escape or win. Fell Toriel, Asriel and Toriel –accompanied by both the Mettatons- took care of the monsters in the refuge –as they termed it- and tended to their needs.

At last, Sans, Gaster, Alphys and her Fell counterpart took to solving and figuring out a plausible solution to the disaster at hand.

And that is how they were where they were at that moment. Racking their cerebrum for doable explanations for the sudden disruption in Time and Space, and the Anomaly.

"To destroy it, of course"

Sans barely heard the Fell lizard's deduction, instead, he stared at the steel walls that were scintillated by the soft glimmer of twilight that shone through the semi-draped windows. His mind was blank, not likely of him at all. Sans always had something on the other on his mind, be it worry, anger or puns, he always had something to wrap his attention around. However, today was different. Actually, if he were honest with himself, all the recent days were. They seemed to float by so hazily, so much so that he did not even take notice of the events at times. He recalled this once where he woke up in the dead of the night, proceeding to wake Frisk up –who slept in his room now that she had confessed her feelings to him, and he to her- and asked her what time it was.

He did not miss that flash of caution that struck across Frisk's face or the lingering worry in her gestures as she held him close that night.

And another time where he woke to a loud sound of a scared gasp, only to be looking at his Gaster Blasters positioned towards Frisk, ready to annihilate her to the depths of nothingness. Their mouths were wide-open with blue wisps of magic crinkling inside, desperate to unleash itself upon the sinner. It had scared Frisk so profoundly that she had broken down. Maybe it was not the fear of dying but the memories that the Gaster Blaster scratched out that resulted in her uncontrollable sobs and violent shaking, he had contemplated later while rocking his mate to sleep. He had held her there for long before she looked at him, and flashed the most determined smile she could muster, reassuring him that she was fine and it was all okay. He had gripped her tighter and had let his frame delve into the soft mattresses, with a giggling Frisk beside him.

Stopping his train of thought before it could indulge into the "passionate" exchanges of that night; he turned towards Alphys, who was gesturing to a graph scribbled in her notebook.

"I-I…" she stammered at first, sweat beading her brows, "…was keeping a…uh…track of their energy signatures l-lately…" she continued, fidgeting in her seat; causing her Fell counterpart to frown and tap her foot on the floor in menace.

"T-They seem to be l-losing…" she glanced nervously at her counterpart, as if searching for approval in those hypnotically gyrating spectacles of hers. On the reception of a flat stare, she continued, "…energy constantly…"

Gaster leaned back into his chair, pinning up his chin with his lean phalanges, seemingly lost in thought. His other hand rapt at the desk, clicking a pen in a rhythm that broke the monotony of the gentle screeches of scrawling pens and beeping buttons.

"Of course they would! They are fighting monsters out there. They will lose energy." Fell Alphys scoffed, crossing her arms against her chest in mild disappointment. For once had her alter felt of use, but as expected she could only pick up postulates and axioms from the sea of abstract. How she even managed to become the Royal Scientist badgered her at moments like these. Then again, that Fluffybuns-King Asgore- could do nothing but make wrong choices; and Alphys was most certainly one of them.

"b-but…" Alphys stammered; sweat dripping down her scales, "the loss is constant. For all of t-them…" She completed with a singe of lost confidence in her voice towards the end.

Sans reclined instinctively, "maybe they're harnessing that energy for somethin'?" He spoke absentmindedly, scratching at his humerus. He so wished for a drink, even a sachet of ketchup would do much for the hammering ache in his head. The pain not only bore physical discomfort but also a threatening presentiment. Something similar to what he would feel during RESETs, although this one was not as piercing and was far more persistent.

"Most certainly." Gaster brooded. His pen was no longer clicking and his eyes were no longer wandering to his troubled son. He was focused.

Sans knew that look all too well. He would have added to his father's mental deduction with a few voiced thoughts of his own if it were not for the pain in his head that had escalated to a dangerous buzz. With his quivering phalanges, he gripped the corner of the sharp metal desk. He had to excuse himself out of the room.

However, before he slid in a tactful pun and made a run for fresh air (he had not needed much air earlier, but at the moment, the air had felt thick and stick with humidity, and his bones had seemed to sodden from the same.) Undyne staggered in, clutching her right leg with a painful clasp; the neck of her black blouse soaked with sweat and her hair rustled to a shaggy mess.

Her breathing was nipped with exhaustion and her eyes narrowed in grieve. If not for Sans' eye-socket for details, he would have missed the tears that had brimmed up in her eyes, blurring the amber of her orbs.

Sans' breath stuck in his throat as he looked at the panting fish, his 'pupils' contracting to invisible pinpricks as he waited for her to speak.

"Sans…Papyrus…"

He clenched his fists, the tips of his boney phalanges scraping against his metacarpals in unsaid trepidation.

"…Papyrus has fallen."