Temmageddon.

  Deep in the underground, just near the end of Waterfall, lay a secret cave populated by a strange creature known as the "Temmie", their small hidden area named, befittingly, Temmie Village. A dog-cat hybrid with grey, frazzled hair, and a light blue shirt covering their chest. They lived in harmony, with no real leader or societal structure, but with no real alignment towards good or evil. They simply exist, often striking up a random conversation with passersby and extending long arms of friendship.
In this town, there resided the Tem Shop, run by a Temmie named 'Temmie' who tended to the Tem Shop. This Temmie looked no different from other Temmies, aside from their shirt which bore bright yellow stripes on top of the blue shirt. Temmie was a good shopkeep, and those lucky enough to find the Tem Shop often found their primary use as a way of discarding unwanted garbage from their bags, as they were the only shop in all of the underground that not only sold goods but purchased goods from others.
Often Temmie would plead for their customers to hand over their goods, offering plenty of G's for their worthless knick-knacks and curios. Despite the strange and frankly unprofessional way of conducting business, somehow the Tem Shop stays afloat, with Tem able to spend any amount of G with little worry or trouble.
However, with such an abusable policy, would grab the attention of a different shop owner who sought to obtain as much G as they could. Namely, a certain Spider...

Temmie was beginning to close up shop for the night, off to go to their loving Tem family at her wonderful Tem home, she was suddenly greeted by an unexpected visitor. It was a purple lady wearing a strange, red outfit. She had six red eyes, all staring down at Temmie with a calming expression.
"Hello, dearie. Mind if I come in?" The Spider Woman asked, standing just outside of the door. "I have quite a remarkable business proposal to share with you."
Temmie nodded with a big smile, rushing back to their place behind their cardboard stand. "yaYA! spODER... tEM FREND!"
The woman was taken a bit aback by the strange energy this creature exuded, yet regardless, she entered. "Ahem, yes. My name is Muffet, I run the Spider Bake Sale in Hotland. I've been told by my spider friends you're quite generous with your money, and--"
"yaYA! TEM... haz LOTa muNZ!" Temmie interrupted, looking around the room, her attention fragmented into several directions.
Muffet took a breath and smiled. "Well, I decided to pay you a visit, and offer you a proposa--"
"we haz... LOT of tinGS! Joo want... TEM FLAKE?" Temmie spouted out once again, not seeming to be listening to Muffet at all.
Muffet had no idea what a Tem Flake was, neither was she interested. At this point she was getting quite frustrated, so she decided to pull out her offering. It seemed to be a massive bag filled with cakes, cupcakes, brownies, croissants, cookies, all sorts of baked goods. "Alright, I'll just cut to the chase. We are willing to offer you this large amount of sweets and pastries, all made by my wonderful spiders, in exchan--"
"WOA!! u gota... LOTS o TREET!" Temmie's tongue flopped out, drooling all over the place as she stared in awe at the sheer amount of food that was there. "hnnnn... i gota have dat lots o treet... but i gota pay for colleg...."
Muffet giggled. "Well, perhaps we could work out some kind of arrangement, hm?"
"hnn...!! tem always wanna lots o treet...!" Temmie winged, biting her lower lip, trying to keep herself from caving into her desires. However, being so weak-willed, she fell into greed, and stated. "tem by lots o treet for..... 9G!"
"N... Nine? You're joking, right? My spiders worked tirelessly for all this, and you only offer a measly NINE G?!" Muffet growled, stamping her foot on the ground as she stood her ground.
"S-so... n-no wan gib to tem...?" Temmie asked, her mouth trembling with sadness.
Muffet shook her head. "Absolutely not."
"b-but...." Temmie pleaded. "p!!!!!"
Muffet stood resilient, though. "I know my treats are worth more than what you're offering."
"okies... tem offer... 15G!" Tem stated.
Muffet was finished here. She turned around and was about to leave when she saw that the entire bag of treats was surrounded by Temmies. She tried to walk towards them, but they all stared at her, watching. Menacingly.
"You will regret this." Temmie stated, smiling behind her back.
A bead of sweat dripped down Muffet's neck.

Temmie was now the proud owner of a big bag of delicious spider pasties, she had to move all of her other decorations in the shop to make room for it. The bag was nearly ten times the size of herself and took up half the space in the Tem Shop.
"Mmm... That is... Lot o treets! Al for tEM!" Temmie salivated in the absolute majesty of such delicious food sitting just in front of her. Without further ado, Temmie jumped into the bag and started eating all she could.
The feasting was absolutely ravenous, bits of dough, frosting, and sprinkles getting everywhere, but a majority ending up in Temmie's maw. She gulped down all she could, her body experiencing almost immediate weight gain as she started to blimp in size. Her shirt rode up her body as she slowly expanded into a massive heap of white, pillowy flab. Her butt was becoming more noticeable as well, two clear lumps of fat showing two large asscheeks filling out her body. The larger she got, the more gluttonous she became, feeling an incessant need to keep eating.
Her entire shop was being flooded with her own fat, anyone who looked in would only see two massive buttcheeks attached to an indistinguishable mass of pure white fat filling the room slowly as the sounds of munching and consumption echoed through the halls.
Temmie was more than ecstatic, the feeling of such unrestrained gluttony making her feel all kinds of happiness within her. She loved her jiggly and flabby she was becoming, how delicious all the food was, and the knowledge that she was far from finished. This would only be a taste of the growth to come...

"dose tem... feel ground shake?" Temmie asked Temmie.
"tem not now wat tem meen... ground not shake." Temmies replied to Temmie.
"oh. dose tem... now abot beeg ball behind tem?" Temmie asked Temmie.
Temmie turned. "O. dat beeg tem. eet lot o treet."
"o. okies." Temmie responded to Temmie.
Temmies were both then squished beneath the growing fat of Temmie, who was slowly filling the entirety of Temmie village with her fat. All the Temmies ran in... What could be ASSUMED was fear, but considering they only ever express one emotion, it was hard to tell.
"ruNZ... ET TEM-MAGEDON!" Temmie yelled out, as they began enveloped in one of the several fat folds of the ever-expanding Temmie consuming a nearly endless supply of spider treats.
Temmie nommed on all of the treats she could, thankfully placing the bag in such a way that all of the treats would pour into her mouth with little movement or interaction, allowing her growth to proceed much more quickly and efficiently.
"yezz... tEM GIT... BIG! be bigest tem... EVAR!" Temmie stated as she continued to eat, her speech in no way being impeded by her constant binging. The walls began to collapse around her as her weight spread further, pouring into the rest of Waterfall. She felt plantlife and mushrooms being squished and crushed underneath her immense flab, as well as crystals being shattered. Clearly, nothing would impede Temmie's immense growth.

"I'm sorry for not getting a chance to spend time with you more, Blooky," Mettaton told Napstablook, as the two overlooked their snail races.
"it's fine... I'm used to being forgotten" Napstablook responded.
"Oh, don't be that way, Blooky! We're together now, isn't that what matters most?" Mettaton asked.
"I suppose, but... hey, what's that?" Napstablook asked, noticing a herd of monsters charging in their direction. A combination of Temmies, Aaron, Shyren and even Gerson running away from the Mushroom Area leading towards Hotlands.
"ruNZ! BEFOR TEM CONSOOM U!" Temmie shouted at Mettaton and Napstablook.
Suddenly, the two turned to see a massive wall of white, fluffy flesh shoving its way through the corridor to make it's way quickly towards them. The squishy wall of fat was encroaching swiftly, the walls surrounding it crashing down to make room for the rest of the massive blob.
"Oh no! Run, Blooky, before we get cru--" Mettaton paused for a minute, looking at Napstablook for a moment before changing his phrasing. "W-well, before I get crushed, at least."
"I don't think I would mind being crushed... oh well..." Napstablook said, before following Mettaton out of the Waterfall area.
Temmie soon collapsed the entire living area of Waterfall and was moving on to the rest of the Underground, covering as much ground as she possibly could, still not feeling the slightest bit full from the constant gorging.
More and more of the delicious pastries fell into her mouth, and like a black hole of gluttony, she ate it up without hesitation, an infinite stream of deliciousness overwhelming her entire body. It seems she was intent on covering everything in the underground, and perhaps even more, all enveloped by soft, pudgy Temmie!

"That giant, white blob is getting closer!" Undyne shouted from within Alphys' Lab with several other survivors from Waterfall. They were all watching from Alphys' cameras surrounding the Underground as Temmie continued her gluttonous rampage. "We need something, anything to stop this!"
"D-don't worry, I have something that might help," Alphys reassured, as she rushed over to her computer panel and began typing in some numbers. "I've recently outfitted the lab with a freezing laser, it should stop this massive blob right in their tracks!"
Undyne grasped her fists together, impatient and wracked with anxiousness. "Well then, do it already! I don't wanna end up a fish pancake!"
"Are you saying you couldn't fight this off yourself, Undyne?" Alphys asked.
Undyne began to sweat. "W-well, I would, but... I don't want to hurt any monsters!"
Alphys nodded. "A-alright, then. Well, here goes nothing then..."

From atop the laboratory, a giant laser began to arm itself, pointed directly at the ever-expanding Temmie. The weight was approaching dangerously close to the laboratory now, her fat not even being affected by the extreme heat or lava. The monsters from within the lab could see the ground was being covered and consumed by the ever-growing weight of Temmie as they continued devouring more and more.
The laser began to power up and shot directly into Temmie, a bright blue laser blasting directly onto Temmie, causing her to glow blue momentarily before slowing her growth to a halt altogether.
Absolutely frozen, as Alphys predicted. She didn't make a single move. Just as it seemed celebration was in order, and all the monsters started to cheer and dance, a rumbling came from Temmie's stomach.
Soon enough, Temmie exploded with fat, the lab bursting apart into pieces, the monsters being flung about and into the air as the massive Temmie resumed her goal of filling the entire Underground. Several large clumps of fat widened and thickened as she grew larger, her head only becoming buried deeper as she ate more and more.
The laser merely tripled the rate at which Temmie grew, no other side effects whatsoever! She soon covered all of Snowdin, the Capitol, and every monster who resided there, showing little remorse for her monstrous voracity. She saw only endless white fields of her own growth, forgetting a time when she was barely a few feet tall, now well on her way to outgrowing everything in the Underground, making herself the largest monster known to mankind.

Asgore could only watch solemnly from his distance castle, as the land he once knew was replaced under a mountain of snowy fat. He was willing to accept this fate, as his vision went white, then black, then the sensation of soft, doughy fat covered every part of his body.
Tem realized something as well, as she crushed Asgrore's castle, the six human souls were consumed as well, giving her nigh-godlike powers. With these powers, she first lifted the bag up and made it feed her indefinitely, a truly endless supply of delicious treats, neverending fattening from the bag of food. She also made the food that came from the bag five times larger, thus making it much more fattening. With this, the true feast began.
Temmie was met with a barrage of giant cakes, reaching nearly 20 feet in size, all stuffed directly into Temmie and her growing body. She rose higher and higher, pressing against the barrier and Mount Ebott itself, unable to keep herself from eating. It all tasted so good, the growth was so intoxicating, and it wouldn't be long before the entire place shattered, and Tem would hopefully be satisfied...

Yet that would be hopeful thinking. For even after the inevitable happened, Tem's massive, indomitable body burst free from Mount Ebott and was exposed to the world, her body much larger than the previous mountain, she still had the taste to eat even more.
"Tem need moar... moar treet... tem no big enoff!" Temmie complained, the absolute typhoon of impossibly massive pastries not leaving her any room to breathe or speak, yet still, she could do both of these things without any trouble, rapidly destroying the land around her. Cities were crushed, forests leveled, even as the sea retreated as Temmie continued growing. Several fat folds upon fat folds upon fat folds could be seen, thicker than the widest skyscraper.
Temmie's butt was crashing into another continent, while her belly spread far across the rest of the land, soon reaching the ocean. As Temmie grew she only used her powers to make the food more fattening, keeping her growth steady as ever, allowing her to keep growing for as long as she could.
"big... biger... biger!!" Temmie moaned, still eating extraneous amounts of food, her head nearly completely submerged. She was having the time of her life, only wondering when she would float off this planet and be rid of it forever.

Yet Temmie would be shocked as she soon found herself floating off the rock known as Earth, and realized only as Earth was getting smaller and smaller compared to her vast, immense obesity that she understood just how big she was... And how much she wanted to get even bigger! She opted to summon several more bags, cloning the original bag, and poofing in several new ones, all of whom were dedicated to fattening Temmie more.
As Temmie got fatter, her belly trapped Earth, including all other planets, between their folds, crushing them like mere peanut shells under the boot of a cowboy at a steakhouse. Temmie couldn't care any less, though. It seemed she was having plenty of fun, even if she was now the last living creature in the universe, a Temmie's motives are never clear. But as far as Temmies go, it seems this one is fairly satisfied.
An endlessly growing furball filling up every inch of space. Everything in reality is replaced Temmie. Every atom, every molecule, every inch of the bound of this universe is simply Temmie. No planets, no galaxies, no solar systems, no stars. Only Temmie. Just Temmie.

The fattest, cutest end to the universe there ever was.