On a clear and sunny day in Ponyville, Rarity made her way towards the castle of Friendship, watching the light reflecting off its large crystal spires. Lately, everypony had been busy, with the gang only managing to gather together yesterday. Although during their gathering at Sugarcube Corner, they’d noticed that Twilight hadn’t been present. In fact, none of them had seen her or Spike for a while. Rarity had been the one to volunteer to check up on her as soon as she found time.
With the main doors unlocked, Rarity let herself in and looked around the expansive lobby. “Twilight, Spike!” She called out, making her way in further towards the main stairs. Naturally, without a reply, the mare assumed that Twilight was simply entranced with a book and hadn’t heard. Heading along to the library, she was confused about how quiet it was. Surely Spike would have come out to see her. Pulling open the double doors to the library, she quickly noticed the scattered piles of books around the room with no organization to be found. Sitting in the middle of the ocean of books was Twilight resting upon a red cushion, idly flipping through her current book. The princess’s mane had a few hairs out of place but otherwise looked rather calm. “Twilight, there you are.”
Twilight blinked and looked up with a smile. “Hello, Rarity,” she replied, peering back at her book a moment later. She seemed a bit distant, almost like her thoughts were elsewhere.
“Where have you been? We haven’t seen you for a week. I know you love to do research, but hiding away is a bit extreme, don’t you think?”
Twilight smiled back as she sprung to her hooves. “Oh, but I am doing something important. I’ve decided to reread all of my books.”
Rarity glanced at the numerous shelves and just tried to fathom how long it would take to do such a task. “But why?”
“Because I need to,” Twilight said, looking at Rarity with a sparkle in her eye. “I can’t afford to stop for breaks, or it’ll take me forever. Maybe you could help me?” Without delay, the princess’s horn started to glow with a grin widening on her muzzle.
“Help?” Rarity asked, finding Twilight’s magic surrounding her before she could ask further. What happened next occurred so fast that the fashion mare barely had time to take it all in, stuck being utterly bewildered. The first thing she noticed was the sense of vertigo as she rapidly sank towards the floor, her limbs receding into her. Her elegant purple tail began to retract, stretching across her flank and forming a long strip of material. Her legs gave out next as she fell onto her chest with a soft crinkle as her inner core was changed, feeling lighter and filled with something other than meat and bones.
Unable to turn her head, she could only watch as her forelegs flattened out with purple tabs appearing upon the ends. “W-what! How is this helping!” Her new form rustled as she felt her head pressed down and stretched. Her vision was consumed by white for a moment before going dark. However, the fashion mare was still conscious, able to hear and feel from within her new form.
Sounds of rustling plastic fill the room as Rarity was lifted off the floor. Hovering in the unicorn’s magic, she heard Twilight speak, “Mmm, now I don’t have to worry about bathroom breaks.”
The next thing Rarity felt was a tail pulled through a hole she had and the embrace of body heat as she fitted onto the royal behind. A shocking realization struck her as the tips of what used to be her hooves were secured to a flat purple panel. “I’m a diaper! No, no, this can’t be happening!” The sounds of her crinkling body echoed across her figure as Twilight shifted and rubbed her butt cheeks against the material. If possible, she would have blushed as the princess laid back down and pressed her flanks firmly into Rarity. Rarity’s cutie mark was now also printed upon the seat of the diaper in full view.
“Change me back! I don’t want this!” Rarity helplessly shouted in her head, somehow knowing that there was no way for Twilight to hear her. The frequent brushes against the princess’s marehood whenever she’d shift reminded the mare-turned diaper of her purpose, whether she liked it or not. The diaper only had a vague sense of time, her only hint of the passage of time being the very faint sound of a clock from another room and Twilight’s page flipping. She did her best to brace for the inevitable, and after half an hour, it happened.
First came the slight shifting of muscles before the tail flicked. Rarity gasped as a trickle of pee splashed against her surface, with a warm patch slowly spreading. “No, no, no,” the diaper squeaked before the stream began to flow along with a sigh heard from Twilight. The hissing sounded like a roar in her ears as she felt herself starting to swell, warming up the more the princess wet herself. Mentally shivering, she had no choice but to take it and was thankful for the lack of sense of smell in this form. The fact that she momentarily found the warmth comforting made her whine internally.
Twilight didn’t seem to mind that she’d just gone to the bathroom into one of her best friends, sighing once more as she finished. The princess’s face scrunched as she shifted at the feeling of the soggy padding before smiling and flipping to the next page of her book. With her horn glowing, Twilight picked up the purple bottle next to her and suckled out the chocolate milk from the green nipple.
Two hours had gone by as Twilight kept up her reading crusade, occasionally shifting about to wake up any sleepy limbs. This, of course, made her padding crinkle squash around, having cooled since her accident. Rarity, unfortunately, found out that her current form didn’t seem to let her fall asleep to wait out this weirdness. She found if she focused hard enough that she could go into a zen-like state. However, any time Twilight moved, she would stir back awake. The cold damp material felt clammy and spongy Rarity had no clue how Twilight could stand it.
Things quickly turned from weird to worse as Rarity felt Twilight adjust herself before hearing a faint rumble. Moments later, the princess let out a fart that vibrated the plastic. Rarity squeaked at the feeling letting out a mental whine in horror, “Sweet Celestia, not that! Anything but that!” Not that she could do anything about it as Twilight’s tail hiked up, followed by a light grunting. The first warm log pushed out into Rarity as she started to expand. She mentally shivered, feeling it slide down and spread around before reaching the seat. The mess weighed her down and shifted around like heavy mud as more soon came down to join the first. The diaper sagged as Twilight adjusted her legs before unloading with one more push and sighing once done. The princess lowered her weight onto her full diaper, spreading the mess further. The fashionista whined to herself, feeling so dirty and used. To say she felt demeaned would be an understatement. A strange tingle of contentment and pleasure washed over her as the mess warmed up her core. “N-no, I am not enjoying this! I feel disgusting, so warm, so heavy.” Despite herself, she slowly moaned after a few shifts of Twilight’s bottom. The squashing felt similar to being smothered in a mud bath. As the minutes ticked by, Rarity was left to ponder just how long she’d be able to last and just how long she would be a filthy diaper.
An hour passed as Rarity eventually stopped whimpering from the squishy sensations in her core, disconcerted she was starting to cool as time went on. She felt Twilight shifting every so often, which caused the mess to fill any missing spots. “How is Twilight tolerating this?! No amount of baths is going to make me feel clean again!” Rarity mentally shouted, suddenly feeling a fresh warm stream of pee flood into her, warming her back up, leaving little to no dry surface left. What made it disturbing was that she realized she felt less disgusted about it than the last time.
About five minutes later, Rarity felt Twilight lean onto her side, pressing her hoof into the diaper. “Hmm, I guess I should change.”
“Change? Does that mean she’ll turn me back to normal?” Rarity hoped as the magic hum of Twilight’s magic started as she felt it surround her form. She definitely would have some very unladylike words for Twilight when she was back to being a pony. Another worrying thought came to her mind as the seconds ticked on. “What if she takes me off and tosses me away! No, no, Twilight would never do that, right?” But instead of feeling the tabs pulled off, she felt the aura of magic around her increase as she heard Twilight grunt in concentration. Rarity’s entire soiled form rippled with magic in waves and pulses lasting a few seconds. With a small flash that Rarity couldn’t see, the fashionista-turned-diaper suddenly felt clean of all filth, now firmly pressed upon Twilight's backside.
Giving a few shifts and hearing the diaper crinkle, Twilight smiled,” Mmhmm. Good, back to reading.”
Rarity let out a relieved sigh at not being thrown away like trash while also being glad she was now clean, at least for now. Her relief, however, was short-lived as she moaned faintly in embarrassment as the situation sank in. If Twilight’s magic could keep cleaning her up, how long did that mean she’d be stuck being the princess’s toilet?! Her form already tingled and felt less strange after a few hours, and she wasn’t sure how she would be in a day or more. What could happen if somepony else came to check up on Twilight? What would Twilight turn them into, a onesie, a teddy bear, or a pacifier? Who knows what she’d do in the cooky state she was in? All Rarity knew for sure was that she was in it for the long haul. She just hoped she could withstand it.
Author's Note
Phew, this story stinks. 
