The only chapter
It was too late for her, and she knew it, but that didn't stop her from trying. Throwing open the door, Twilight Sparkle raced through the library. Her eyes were wild, her heart pounding in her throat. She could feel it welling up inside, a heat that made sweat drip from her brow despite the relatively cold temperature outside. Her hooves ached, her stomach twisting painfully with every step, but she managed to climb the stairs to the bathroom.
She didn't have to shit, though the pressure on her bowels was enormous. It wasn't just regular pressure, either – She could feel the gas roiling inside of her, poking and prodding like a sapient creature searching for a route to escape from her constricting body. A twinge of pain shot through her midsection, causing her to grunt as her stomach lurched outward another inch. The pressure on her backside increased, sending out a curl of black smoke bearing the stench of a well-rotted fish.
Reflexively pinching her ass cheeks, she stumbled through the bathroom door and waddled over to the toilet, hopping atop the porcelain throne without so much as a thought. It was a mistake. No sooner had her fuzzy buttocks touched the plastic seat, than a wave of oily stench blasted from her plump, aching ponut.
“C-Celestia, help me...” She moaned, knowing all too well that was impossible. Even if the princess could behold her student's terrible fate, there was absolutely nothing the alicorn could do to save her, save to incinerate her and all around to prevent the infection from spreading. Whatever corruption was taking root in her soul, Twilight retained a certain glimmer of self-preservation.
“It stinks,” She muttered pointlessly. Of course it stunk. She'd conjured a servant of Nurgle, what was she expecting, lavender and roses? And that demon was inside of her... Bile rose in her that, and it was only with extreme effort that she managed to avoid puking right then and there.
The heat was overwhelming, sweat rolling off her body in waves as she pushed out another wave of horrific wind. She needed something to cool down – Impossible, but the need was gnawing at the part of her brain that was still equine. With a grunt, she heaved her increasingly-heavy body from the toilet, grimacing as her bloating ass cheeks squished against the seat. In her desperation, her first instinct was to try the bowl – But it was full of black, oily goop, pure pestilential corruption. A shaky hoof raised up to flush, but not all the goo went down the drain, the remainder quickly corrupting the clean water flowing in.
In the back of her mind, she couldn't help but worry about the impact of the corruptible waste in an unsuspecting habitat, but that no longer mattered either. Poking a hoof into the so-called water, she cursed, more from instinct than pain. The oil was as thick as sludge, and hot like a simmering kettle! And all that came out of her... She wanted to puke again, but instead, she scrambled into the tub, hauling her creaking, wind-filled belly over the edge.
Sparks flew as she attempted to use her magic on the shower hose, a numb feeling spreading throughout her head. No doubt, the infection had finally reached her brain. This was it, then, no way back. Even if there was a cure, she was about to start suffering irreparable neurological damage. With luck, she might even remember being an equine, once, instead of a massive stinkblimp.
With the last ounce of energy, Twilight twisted the knob, bringing a rain of cold water down upon the purple princess. She let out a ragged sigh, mixed with a moan of pure ecstasy as the infernal heat roiling through her core began to decrease, turning into nothing more than an uncomfortable, throbbing warmth.
“S-so good,” She moaned, opening her mouth to catch the spray of water on her tongue. She didn't notice how deep her voice had become, at least an octave lower than usual, with a scratchy quality like a paper bag crinkling as it's blown down the street. Such a minor thing barely registered on her list of priorities, even lower than her belly, which slowly stretched to accommodate the rushing water.
All too quickly, however, the relief ended, as the temperature started to ramp up. At first, she thought she'd accidentally knocked one of the knobs, but a prod with her ragged hoof showed that only the cold was on. Yet, even as she guzzled down the spraying water in an attempt at quelling the infernal heat growing within her, the temperature was steadily climbing, the sharp chill of the cold water swiftly becoming lukewarm as it torrented down into her swaying gut.
“Please, please,” Twilight moaned as the water warmed up. She could feel her stomach churning, gallons of water sloshing around within a container that was never meant to hold so much. The heat was rising there, too, every last remnant of the cold slowly turning to what would otherwise have been a pleasant warmth, now an unwelcome reminder of her terrible fate.
She clambered out of the tub, struggling to haul her belly over the rim before collapsing on the floor. Even the cool tile felt uncomfortably warm against her body. Excess water rolled down her flanks, mixing with sweat in a puddle of yellowish filth. She tried to shut her eyes, to meditate, find her happy place, but there was no such reprieve, as the water within her reached boiling point.
Steam rose inside her belly, stretching it out even more. Pain shot through her body, red stretch marks appeared across the purple surface as it strained to contain the churning, bubbling water within. She tried to keep her mouth shut, but within moments, the pressure became too intense. She felt like a bomb, ready to detonate at any moment. Then, she did.
Her jaw wrenched open, letting out a blast of humid, reeking steam, tinged green with the corruption roiling within her very soul. Her ass was next, the fattening donut-ring of her pucker widening to allow a wave of overwarm stench, flecks of black, corrupt filth spraying across the wall and the side of the bathtub, before being washed down the drain by the still-hissing shower. Normally, she'd worry about such a containment breach – If the local treatment plant didn't clean it up, who knows what kind of horrors those few droplets could wreak?
But her mind was in no condition to worry about such things. She could feel the filth gnawing away at the corners of her mind, eroding the very core of her mentality, her personality – A principle of higher mathematics disappeared, leaving an empty void that was swiftly filled with purest corruption. Memories of her past, her friends, her family, of sunny days and cool waters, were swiftly turning to thoughts of filth, of consumption and excretion in amounts never before seen.
The feelings were overwhelming, but Twilight managed to haul herself upright. Her belly hung mere inches from the floor, the sheer width forcing her to take a wide stance. She tried to think, but her foggy brain wouldn't allow it. She knew she had to do something... And that there was a reason for it... But beyond that, no complete thoughts could break through the haze of mind-melting heat.
Glancing around, she caught sight of her face in the mirror – Barely recognizable as the alicorn princess of friendship, Twilight's face was bloated with fat, cheeks billowing out like pimple-covered sandbags, half a dozen sweaty chins cascading down her tire-like neck. Her ragged mane quivered like a mop in the wake of her windy expulsions, strands of dirt-encrusted purple hair drifting to the floor.
Even as she watched, her glazed eyes could see acne sprouting across her formerly-smooth face, growing to the size of plump grapes before exploding, sending a spray of grayish puss across the room. Deep down, what was left of the old Twilight was horrified at what her body had become, but that was fading quickly. Her very soul was being eroded, imbued with the unholy stench that permeated the room, spewing from every orifice like smoggy poison.
Twilight's belly gurgled and expanded, a jolt running down her back it made contact with the cold floor. It wasn't cold for long, as the heat from her belly transferred to the tiles, making the puddle of sweat that inundated the floor begin to simmer. Steam rose up, obscuring the mirror and making the room seem like a sauna.
Filth swirled through the air in thick strands, spraying from her mouth and ass like twin fire hoses. Just a drop would be enough to utterly destroy a pony's mind and soul, corrupting them into yet more monsters. Thankfully, the door and window were sealed shut, containing the pollution to the bathroom. The seals wouldn't last long, of course; Even now, the corrosive gas was eating slowly through the rubber lining, allowing the odd puff of hellstench out into the world. Hardly a whiff, yet enough to plant the seed of slobbishness within anypony unfortunate enough to catch a breath of the stuff.
Suddenly, there was a gurgle, and Twilight's belly lurched outward another few inches, separating her hooves from the floor. Before she could react, her mouth lurched open, releasing sound between a belch and a roar, along with a blast of pitch-black gas. It wasn't just a belch; The gas seemed to swirl through the air, taking a shape not unlike liquid. The stench in the room increased significantly, though the air was already toxic enough to instantly suffocate any creature foolish to venture inside without a gas mask.
The smoky liquid drifted through the air, slowly congealing into a shape vaguely similar to a pony, albeit massive, easily twice as fat as Twilight with a belly that would've grounded it were it not floating several feet off the floor. Two eyes glowed with blue flame, and the mouth opened, letting out a wave of horrendously filthy air that caused the alicorn's eyes to sting.
Twilight squeezed her eyes shut, tears appearing at the corners; Those tears slipped away, replaced with a black, oily substance resembling the secretions of the stench-demon before her. When they opened again, the mare's magenta eyes had been replaced with blue spheres, glistening with the infernal fire that burned within what was left of her corrupted soul.
“There, isn't that better, my little stink-slut?” The monster's wispy arm wrapped around her muzzle, “You should've known this would happen. There's a reason I was damned to Tartarus, you know.”
“I should thank you, however. This world has been without my corruption for far too long. As thanks for releasing my from my bonds, I grant you life eternal, that you may enjoy and spread your filth unto the end of time.”
The blob didn't respond, too busy spewing liquid gas from both sewer-pipes. Giving his acolyte's backside a gentle caress, the demon couldn't help but smile. With one princess out of the way, the rest of Equestria would be easy pickings. The rivers running brown, the dead trees and abandoned houses, the few bloated survivors splattering filth across what remained beneath the poisoned green sky...
He could hardly wait!
“Twilight?” A rough, male voice called from downstairs, “What in Celestia's name is that smell?!”
The mound that was Twilight let out a gurgling groan, her glowing eyes screwing shut for a moment. Strain was evident on her face, and for a moment, the demon felt a twinge of concern – Just a twinge, nothing more. After a moment, there was a wet splatter as a wave of black sludge erupted from her backside, spraying across the wall and flooding the bathtub, only to run down the drain.
The demon reached out a shadowy tendril, wiping off a little of the filth and giving it a savory lick. He snorted. Without so much as a final glance at his acolyte, the demon's vaporous form drifted under the door and into an unsuspecting world.