Starlight Glimmer starts absorbing cutie marks from her collection into herself with her magic. Aside from sexually stimulating her, it makes her grow abit taller and bigger, even growing wings to become an alicorn. Thinking that if she absorbs even more, she'll get even more powerful, she gets the rest of them, including the main six's, and absorbs them all as well. However, after this, instead of making her more powerful, it causes her to gain weight until she's practically helpless.

“You didn’t think that you could actually win, did you?” Starlight Glimmer sneered at the ponies gathered in front of her. Her horn was glowing, summoning up magic to unlock the Mark Vault behind her.

“You’re through,” Twilight Sparkle shouted. “Come quietly, and nopony has to get hurt!”

“Please,” Starlight snarled. “I have one more trick up my sleeve!” Her horn flashed blue.

Before any of the other ponies could react, every cover on the Mark Vault opened, the enchanted glass sliding open with a crash. Several of the ponies in the crowd looked hopeful, and most looked confused, but not a single one of the marks made its way back to its original owner. Instead, Starlight Glimmer chuckled to herself as a dozen or so flew down to surround her in a halo of flashing lights.

“None of you will ever get your Cutie Marks back,” Starlight growled. “Because I’m going to take all of them - forever!”

Twilight Sparkle threw a magical bolt at the same second that the last Cutie Mark was absorbed into Starlight’s coat - and gaped as it bounced off, falling harmlessly to the floor. Starlight glanced down at herself in disbelief, then lifted her head to the skies, cackling madly.

“I can feel it! I can feel their magic coursing through me! Make me stronger!”

The crowd watched in horror as Starlight began to grow - first, her legs and neck, and then the rest of her body, swelling in height and overall size until she stood at least as high as Celestia. Twilight Sparkle gaped as Starlight let out a roar of pain and pleasure, two lumps rising on her sides and erupting into a pair of alicorn wings. All throughout her transformation, Starlight’s laughter continued unabated.

“Oh, Celestia,” she said, her mad giggling finally petering out. “No - why should I swear by an inferior being? I’m bigger than her - stronger, too, I’ll bet.” She sneered down at Twilight, who now could barely even reach her shoulders. “And you thought that you were so special, you little alicorn bitch.”

“This - this can’t be possible,” Twilight sputtered.

“Oh, you’d better,” Starlight said. She suddenly moaned as a wave of pleasure rode through her, surging from the tips of her hooves all the way to her larger, powerful flanks. “Oh, my. Being big feels…good.” She shuddered, a few drops of marecum dripping from her giant marehood onto the ground.

“And if I’m this powerful just by absorbing a few paltry Cutie Marks,” she said, sneering down at the crowd, “then how big will I be when I absorb them all?”

“No!” Twilight hollered. “Stop her!”

But Starlight Glimmer only laughed as more and more Cutie Marks surged from their cases to land on - and be absorbed by - her body. A surge of magical energy exploded from her body, violently throwing each pony rushing her to the ground. “Yes,” she moaned, her chest swelling. “Bigger. Bigger!”

“No way,” Rainbow Dash breathed as Starlight Glimmer grew and grew, her head reaching an easy eight - no, ten - no, fifteen feet in height! “What the hay are we supposed to do to stop her?”

Twilight could only stare up at Starlight Glimmer’s growing form in horror. By now, the newborn alicorn was a good eighteen feet tall, her enormous hooves almost as big as a whole pony, and heavy enough to crush any one of them easily. Her horn had almost reached the top of the cavern, and Twilight was almost scared to think of how much raw magical power Starlight could use. Meanwhile, there was no easy distraction from the allure of Starlight’s massive, winking clit. More and more marecum dripped onto the ground beneath the giant mare, whose very growth seemed to be bringing her to orgasm.

“Wait,” Applejack said. She put a hoof on Twilight’s shoulder. “Something’s happening.”

“Yeah, she’s getting even bigger! We’ve got to find a way to stop - “

“Look again.”

Twilight squinted. Then a grin spread across her face.

“Bigger!” Starlight roared, her very voice shaking the cavern floor. “Bigger! Make me bigger!” More and more Cutie Mark poured into her, their glows filling the cavern with light - but she didn’t seem to have noticed that she wasn’t growing any taller. instead, Twilight realized with growing glee, she seemed to be growing…sideways.

“Bigger,” Starlight panted, riding out her latest orgasm as her body twitched and convulsed. “Bigger. Bigger.” And it looked like she was getting her wish after all. Pockets of fat began to bubble up on her sides and on her belly, swelling in size and weight with each Cutie Mark she absorbed. Soon, she had a massive, two-ton belly that hung low between her legs. Sweat dripped down her second - and soon, third - chin as her tree-trunk legs struggled to keep her porky body aloft.

“Looks like she bit off more than she could chew,” Applejack said, smirking. Starlight seemed to have finally realized what was happening, an expression of horror dawning across her face as she shook her head back and forth.

“No,” she gasped, her jowls bouncing and the rest of her massive, fat body jiggling with the syllable. “This - this can’t be possible. I - ugh!”

And with an earth-shattering boom, her giant, fattened alicorn body crashed to the ground.