As Starlight ran, she dragged a bubble of pink magic along behind herself. Inside the bubble...a houseplant. A few photos and unusual books. A couple of doomsday and/or brainwashing devices. And the liveliest, most squirmy piece of cargo: a pale blue unicorn with a silvery mane and tail.
Starlight reached the entrance of a cavern, and behind her the bubble-wrapped unicorn's horn lit up, sparking and sputtering and finally venting an explosion of light and heat, bursting the pink bubble.
"Starlight!" the unicorn shouted. "What's going on? Where are you taking Trixie?"
Starlight spun around, facing her fellow pony. "This hole leads down to the mysterious Harmony Caves beneath the School of Friendship. So I guess technically, we're on top of part of the school."
The blue unicorn snorted. "Trixie can recognize when somepony is trying to pull the sheep over Trixie's eyes. What aren't you telling Trixie?"
"Well...I MIGHT have been doing a magical experiment and made a terrible, terrible mistake. But that's not important! What's important is, we need to leave this universe forever! And never look back."
Trixie snorted again. "Who do you think Trixie is, a coward? Trixie doesn't run away from danger...unless Trixie makes sure it's really dangerous first. Traveling across Equestria time and again all by herself, with only Trixie's wits and excellent judgement to protect Trixie. Daring to battle the Alicorn Amulet to win unimaginable power. Telling ridiculous lies about Ursa Majors that nopony should ever be able to catch Trixie in, but if they did it could turn out pretty dangerously. Those are the courageous things a brave mare like Trixie does. Not running away from danger sight unseen, like a frightened filly."
Trixie nodded with self-satisfaction. "So Trixie refuses to run away from the end of the world without even taking a look at it first."
"Trixie...you are so naive, so innocent. Do you really think the end of the world is the worst thing that could happen?"
Trixie turned her back on Starlight, looking away from the cavern.
Off in the distance, a group of short, scrawny gray stallions walked slowly towards the two mares. Nasal, pedantic conversation failed to fully cross the gap, but Trixie recognized some of the tone and cadences.
Trixie spun to stare at Starlight. "Your magical accident...you created an entire herd of Mudbriars?"
Starlight shook her head sadly. "Worse."
"You...what could be worse?" She blinked. "You created TWO herds of Mudbriars?"
"Your guesses are barely even getting closer to the truth."
Trixie lifted a forehoof to rub her head. "Maybe you're just trying to drive Trixie mad. What did you do? What did you DO?" Trixie looked over her shoulder. The Mudbriars had come a little nearer.
Starlight grimaced. "Did you ever hear about what I did, when I had an entire town to experiment on? I thought about cutie marks, and I thought, if only ponies weren't so different from each other, we could get along better, and all be friends?"
Trixie took a deep breath. She looked over her shoulder again, at the slowly encroaching Mudbriars. "You turned everypony in the world into Mudbriars?"
Starlight laughed bitterly. "That's just ridiculous. Obviously you and I aren't Mudbriars."
Trixie peeked over her shoulder again. A voice came faintly on the wind: "Technically, Starlight is correct." Trixie couldn't make out the words that followed, but she could tell there were a lot of them.
She interrupted the distant stallion, to speak to Starlight again. "What did you do?"
***
In one of the dustier, more out of the way rooms of Twilight Sparkle's Crystal Castle of Friendship, Starlight sneezed. Rackfuls of test tubes and beakers and slowly bubbling flagons received a scattering of contamination. Starlight shrugged. "No problem. It'll be fine...probably. And even if the experiment DOES go wrong and causes a serious mishap, Twilight deserves it for not dusting her guest rooms better."
A door opened behind Starlight. "Technically, Spike does all the dusting around here, not Twilight. Also, I don't see any reason you couldn't help out around the house a little too. You aren't even paying rent."
Starlight grimaced. "What are YOU doing here, you obnoxious...I mean, why aren't you following Maud around, or getting somecreature to throw sticks so you can fetch them?"
Mudbriar replied, "Maud and sticks aren't the only important things in my life. They might be MOST of the important things in my life, but not ALL of them. Also, Maud sent me to get a beverage." He inhaled audibly. "Oh, grape sodas! Maud's and my favorite!" Mudbriar used his teeth to pick up a glass cylinder full of bubbling gray liquid. He tilted his face back, spilling the potion down his throat.
"No!" Starlight screamed. "You weren't supposed to drink THAT one."
"What?" Mudbriar asked. But Starlight didn't answer him. She'd already run out of the room screaming.
***
"So what you're telling Trixie is, you were working on a potion to create a magical contagion that would turn everypony in the world into the first pony who drinks it. Hmmph. Talk about poor planning."
"I was still perfecting it! I didn't expect anypony to DRINK it!"
"That isn't what Trixie means. If you were going to turn everypony in the world into the same pony, you should have chosen to turn everypony in the world into the most beautiful and best pony. That would have made some kind of sense."
Starlight sighed. "So you DO understand my plan."
"Of course Trixie can guess your plan. Trixie's marefriend must have wanted to give the potion to Trixie...until somehow, Mudbriar got to the potion first. Trixie doesn't think her marefriend is a complete moron...not quite yet. But Trixie's marefriend WILL BE a complete moron, when she turns into Mudbriar."
"Trixie, look out behind you! Run!"
But the encroaching stallions were already too close. In a swirl of dull gray-brown transformation magic, the two mares changed color, size, and shape.
"Technically, there is no Trixie, not anymore. There is only Mudbriar."
Author's Note
Where Do Story Ideas Come From, Anyway?
One of this story's sparks of inspiration was a thought I had about Bicyclette's story "Trepidation." (link)
I wanted to read more about the encroaching magical disaster.
So I wrote a story about that! As one does. 
