Adagio Dazzle, Aria Blaze, and Sonata Dusk were wandering a cave, a ways outside of the city. They had adjusted to living a life without their siren powers, but hated not being able to control people anymore. The world they had been banished to had gotten ten times worse, and it was all because of Sunset Shimmer. After hearing of more Equestrian magic coming through, they had searched for something, anything, they could use to get their revenge.
Eventually, while exploring a cave, they saw a glowing light, and ran towards it. As they got near, they saw the source of the light: Three glowing stones, nothing like their siren pendants, but definitely magical.
"I wonder what they do," said Adagio, picking up one of the stones.
Aria picked up another. "One way to find out," she said, and she aimed it at Sonata and blasted her with its magic.
...
A few days later, Sunset Shimmer and her friends were having a picnic at the park. Sunset had been exercising a lot lately, leading to more fun times with Rainbow Dash, so Rarity had fashioned a new exercise outfit for her, consisting of a pinkish purple T-shirt and skintight black spandex shorts that went down about a third of the way to the knees.
As the girls were making plans for a big party at Rarity's house tomorrow, the sirens watched from behind a bush. They were each wearing a necklace with a magic stone on it.
"Whatever you do, keep your trap shut," Aria warned Sonata. She knew the silly siren would be tempted to laugh.
Adagio raised her stone, pointed it at Sunset, and cast a small bolt of magic. It hit Sunset directly, though she didn't feel or notice anything. Until a second later.
FFFFFFFFFRRRRRRRRRRRRRT!
All the other girls turned toward Sunset. Sunset blushed and giggled nervously.
Sonata burst out laughing. Adagio and Aria shot her a disapproving look, but luckily for them, their enemies couldn't hear her laughter because Rainbow Dash and Pinkie Pie were laughing louder.
"Sunset!" said Rarity. She sniffed. "Ugh! This is absolutely foul!"
"I'm so sorry," said Sunset, blushing beet red. "I didn't mean to."
Rarity could see how ashamed Sunset was. She remembered the times she'd held it in for hours, and how humiliated she'd have been if she had been unable to in front of her friends. Especially considering how loud her farts were afterwards, once she was alone. "Oh, it's all right. Just... try a little harder to hold it in."
Sunset was relieved that Rarity wasn't holding this against her. Rainbow and Pinkie had stopped laughing, and they continued their conversation from before as though nothing had happened.
"Well, that didn't work," said Sonata.
"I told you one fart wouldn't have much of an effect," said Aria.
"Then let's take the next step," said Adagio, raising her stone. She and Aria had experimented on Sonata with the stones to find out the extent of their powers. Sonata was the only one who wasn't afraid to fart in front of the others, and she found it rather fun. The sirens had discovered that the stones could make a person as gassy as they wanted for as long as they wanted, and had planned to use it to get revenge on Sunset by humiliating her in front of her friends.
Adagio thought, Make her fart for an hour, and zapped Sunset with the stone's magic a second time. If this went according to plan, her friends would be too disgusted with her to go near her again. Except Rainbow Dash and Pinkie Pie, who would never let her forget it.
Sunset groaned.
"You all right, sugarcube?" said Applejack.
"I... think I should step away for a few-"
BBBBBBBBBBBBRRRRRRRRRRT!
This loud, deep-pitched fart was followed by many shorter farts.
Rainbow and Pinkie started to laugh again, as did Sonata from the bushes.
"Stop it," said Applejack. "Can't y'all see how embarrassed she is?" She patted Sunset on the back. "Don't worry your pretty li'l head none. We all fart. It's nothin' to be ashamed of."
Rainbow Dash and Pinkie stopped laughing. They still thought it was funny, but they didn't want to make Sunset more uncomfortable.
"It..." PRRRRRRRT! "won't..." FFFFFRRRRP! "stop!" PPPFFFFT!
"You must've eaten something that didn't agree with you," said Pinkie. "It happens to me all the time! Just wait it out!"
Sunset grunted and let out a longer, wetter fart. "This doesn't seem like it will go away any time soon." She tried her hardest to hold it in, but after about ten seconds, a boisterous wet fart made its way out of her. She sighed. "I'd better go home until this stops. I don't want you to have to put up with this."
"Do let us know when you're feeling better, darling," said Rarity.
...
As Sunset ran home, she farted several times. She'd try to hold it in if she ran by anyone but farted in front of them a few times, causing them to either snicker or shoot her a disgusted look. Once she got home, she lifted her leg and let loose, no longer having to worry about feeling ashamed.
BBBBBBBBBBBRRRRRRRRRRRRAAAAAAAAAAAAAAAAP!
She had to admit the relief felt wonderful, but it didn't last long. Another loud, higher-pitched fart later, she climbed onto her bed. She lay facedown and let out several wet farts. Then she lay on her back and lifted her legs into the air.
PPPPFFFFFFT! BBRRRRRRRT! PPPPPPPPPRRRRRRRRRRRRT!
Her bedroom was starting to smell now, but she didn't mind the smell of her own farts. After laying in this position and farting for five minutes, she positioned herself facedown again with her rear end in the air.
PPPPPPBBBBBBBBBBBBBBBBBRRRRRRRRRRRRRRRRRRRRRRRRRRRRT!
A loud, deep-pitched fart made its way out of her, and several more farts came after. Why am I farting so much? Sunset thought. She feared explosive diarrhea or something was coming, but as minutes went by, nothing came out of her behind but farts.
An hour after Adagio had cast her spell, Sunset finally stopped farting. She waited ten minutes to be sure, after which she texted the other girls saying she was better now. They met up again and hung out as though the whole thing never happened.