Dusk Shine's magical potential transforms him into a towering sex god, growing slowly bigger with every orgasm as he masturbates without cooldown. Shining Armor, so used to being in charge, steps into Dusk Shine's cum-soaked room and makes a final effort toward asserting himself as the 'big' brother.

Shining rapped on the door. “Hey!” he hollered. “D’you mind?”

Normally, he was happy to leave Dusk’s room alone. Even when his little brother got a little more...rowdy on some nights, he’d always been more than happy to keep his indiscretions quiet. But this was just ridiculous. He’d heard nothing but thumps, thuds, and ear-splitting groans across the hall for the past ten minutes. This was just getting ridiculous. Dusk was lucky Mom wasn’t home, or else she’d definitely be throwing a fit.

The thumps and groans stopped. “Come in and make me.”

Shining’s temper flared. Usually, he was an even-headed stallion, but dammit, he’d been studying, and he had a test tomorrow, and what the hay was he doing in there anyway? He was too angry to register that his brother’s voice had sounded deeper, richer than it usually did. He clenched his fist around the doorknob, prepared himself for what he was about to see, and flung it wide open.

It took only an instant for him to realize that he hadn’t prepared himself for this.

Dusk was on his bed, alright. Right where Shining had expected him. But the normality stopped there. The bed, and the rest of the room to boot, was absolutely drenched in white. Shining’s nose wrinkled when he identified the thick, creamy substance that was clinging to the walls, dripping from the ceiling, and even squelching around his foot. But that wasn’t the first thing he noticed.

No; what Shining Armor noticed was his brother.

Dusk Shine was, normally at least, a rather slender, mild-mannered stallion. About a half a head shorter than his older brother, he’d always been more interested in books than anything. But now, he was nearly unrecognizable.

The huge, hulking beast of a stallion sitting on his brother’s bed couldn’t possibly be his little brother Dusk. Shining stared up at him with dawning horror, but the resemblance was irrefutable. Just over nine feet of perfect, bulging stallion grinned down at him from over a pair of pecs that looked like a pair of half-ton weights. His stallionhood, more than half as thick as Shining’s waist and two-thirds as tall, throbbed mightily.

Dusk smirked. “Hey there, lil’ bro.”

A “drop” of precum, easily bigger than Shining’s fist, oozed from the tip. Shining watched it fall, slithering down the side of Dusk’s massive, veined cock before breaking open atop his massive ballsack. He was instantly overcome by a wave of thick, overpowering musk, a scent full of sweat and sex and muscle.

He looked back up at his brother’s face and swallowed. He started to say something, but whatever it was died in his throat along with his courage.

He gulped. “How—” He choked on the words.

Dusk smirked. “Turns out when your magic is strong enough…” He left the last words hanging, then chuckled. “You’re staring an awful lot, little brother. Like what you see?”

“No,” Shining snapped. But… His eyes roved, almost hungrily, across every inch of flesh belonging to his once-smaller brother. Dusk’s grin grew, and he gave a not-so-subtle flex of his arms, his massive hands clenched around his shaft. The biceps alone bulged out larger than Shining’s head, and his enormous stallionhood gave another throb at the motion. Then he resumed sliding his palms up and down his length, his sausage-thick fingers just barely managing to meet around on the other side.

No! It wasn’t fair! How could his brother be so big, so massive, so… His mind rejected the word perfect, but his soul certainly thought it. He’d been working out for months to try and get in shape to join the Royal Guard. He’d been successful, too, attracting more than one look from the girls—and guys, to boot—as his arms filled out his sleeves, his chest pushed out his shirt, and his legs grew thicker and more toned. He’d even grown a bit, hitting six foot two on one scale and eight inches on the other. More than respectable—fantastic.

But next to Dusk, those gains were nothing. Shining licked his lips, then blushed when he took in how Dusk was staring at him.

“No,” Shining said again, a little more breathily. It was getting hard to convince himself, really. “You just—you have to stop. You can’t be this loud or this…” He paused, then held out a hand toward the rest of the cum-drenched room. “This!”

“Oh, really?” Dusk smirked at him then bent over, giving his flared head a long, slow lick. He barely even had to reach at all to do it. Shining had tried to suck his own dick once. He’d made it most of the way, too. But Dusk didn’t even had to try; his massive stallionhood was so thick, so long that it reached all the way up to his chest, sandwiching itself between his massive pecs, rubbing against his muscles as he steadily worked the lower shaft. “And who’s going to make me?”

Shining swallowed. “I’ll—I’ll tell mom!” The door slammed shut behind him and he jumped.

“No, you won’t.” “Dusk chuckled. “She’s not even home.”

“Yeah, but—”

“Man,” Dusk groaned, interrupting him. He had his eyes closed, and was steadily pumping his dick up and down. Thick veins throbbed against his open palms, a steady river of pre providing more than enough lubrication for the sex god to jack off. “You have no idea how good this feels.” One eye peeked open. “Little brother.”

“Stop calling me that! I’m not—”

“Yes, you are.” Dusk was getting into it now, his hips jerking back and forth as he pumped. “Mm. You’re little. So little, compared to me. You’re just a little colt, and I’m bigger than you’ll ever be.”

Shining took a step back, tried the door. It wouldn’t budge. “Dusk—”

“And I’m just going to keep on growing.” Dusk was moaning, the bed creaking louder and louder under his weight. A fat spurt of precum erupted from the top of his dick, easily twice as large as one of Shining’s loads, and hit the ceiling above him. “I’m going to get bigger and bigger, and you’re just going to keep getting smaller.”

He was panting, his arms and legs flexing as he reached his peak. Every impact of his massive thighs and rear on the bed shook the whole floor. Shining stumbled, unable to look away.

“I’m going to outgrow you,” Dusk groaned, “outgrow this room, outgrow this house. I’m going to be the biggest stallion you’ve ever seen. And I’m going to keep you around, a tiny little shrimp, just to compare myself to you, so that everypony for miles around can see just how tiny, how whimpy you are, and just how big, how massive, how fucking amazing I am—”

His hips bucked wildly while his cock swelled. Shining could actually hear his balls churning and sloshing as a massive bulge travelled up his brother’s length—and then Dusk was cumming what seemed like gallons and gallons of spooge, an endless wave of white washing up against the walls and ceiling and pretty much everything else. Only a quick shield spell saved Shining from the same fate. The rest of his room didn’t fare quite so well.

Dusk’s orgasm lasted a good twenty seconds, his enormous cock squeezed against his pecs as he erupted with rope after rope of cum. He roared with pleasure, groaned with joy. And then he started to grow.

Shining watched in stunned awe as his “big” brother got even bigger. His head inched up and up toward the ceiling. His muscles swelled, each of his arms now as wide as Shining’s whole body as his legs approached the size of tree trunks. His pecs pushed out from his chest, swelling, bulging, until it was a struggle for Shining to even look up into his brother’s face.

Then the bed creaked, groaning for the last time, and shattered beneath Dusk’s massive weight. Shining was flung to the ground by the force of the impact. He hit the wall, coughing and wheezing.

Then he looked up at his brother...and up, and up, and up. Even sitting down on the floor, now Dusk’s head was just a few feet away from the ceiling. At first, his “big” brother looked even more surprised than he did. But then a sly, calculating look came over his face, and he chuckled deeply.

“Much better,” he said. He stretched, groaning as his joints popped and his muscles swelled. Then his eyes fell on Shining. His massive cock, now even with his eyes and just as big as his formerly “big” brother, throbbed. “What’s that?”

Shining was powerless to resist as his big brother wrapped one enormous hand around his waist, easily reaching across the room, and lifted him up to eye level. He struggled in vain. “Let me go!”

Dusk laid a single massive finger on Shining’s crotch. “Looks like you do like what you see,” he said. There was an evil grin on his face.

Shining gulped. His boner throbbed.