Princess Celestia settled down into her private hot tub and rubbed her hands on her own shoulders, glad to finally be done with this day. The constant bureaucracies of an entire kingdom had a way of tiring her out at times, despite her infinite cosmic power. She relished the chance to strip down to a little golden bikini and settle into the almost-boiling sun-shaped tub. It was a fair deal hotter than most mortals could handle, but she was a being of near-infinite power – for her, it was barely enough.

The door opened behind her. That wasn't right; nobody should be disturbing her right now.

“Hello, Sister,” a voice said.

“Luna? What are you doing here?” Celestia didn't turn around, irked at the interruption. No doubt the kingdom needed to be saved again or some other nonsense.

A subdued, but still maniacal laugh came in reply. “Not Luna, no ... not exactly.”

That voice... Celestia had heard it once before, long ago. “Nightmare Moon!” She whirled around in the tub, splashing as she turned to face the interloper. “You won't take Equestria from me, not again!”

“Easy, there. I haven't come to take Equestria from you.”

Celestia glared at her. “Then what?”

“I come here to take your hot tub from you!” Another maniacal laugh, not subdued at all this time, though.

Celestia stood up in the scalding-hot water. Her tits bounced under their tiny gold triangles as they splashed up from the frothy wavelets. “But... why? And ... and what could you possibly think gives you the right?”

“Why? Because my own lunar pool has malfunctioned, and the royal plumbers say they shall require no less than three days to repair it!” She reached up and grabbed the fuzzy lapels of the black bathrobe she was wearing. “And as for what gives me the right...” Nightmare Moon yanked the robe open wide and let it drop to the floor.

She wasn't wearing anything beneath it. Nothing at all. Every bit of her glossy black fur was on display, even the thick patch above her ebony slit, but what truly drew the eye was her breasts. She'd done something, that was for sure – they must have been ten times the size Luna normally sported. Each gloriously jiggling tit was easily the size of her head; they were even a little bigger than Celestia's.

“So, a contest of hyper-endowment, is it? You should know better than to challenge me, Sister.” Celestia's horn lit up and focused magic around her own boobs. In only a few moments, they grew significantly. Now they stretched and strained the little triangles of golden fabric, which could hardly even cover her aureoles anymore. The plush flesh of her tits swelled out around the straps on every side, leaving each golden strap in the center of a shallow valley of skin.

Nightmare retaliated with a growth spurt of her own, plumping her boobs to a point where she would be hard-pressed to wrap her arms around them. “You should give up before this escalates further. You cannot win.”

“Never!” Celestia really needed this relaxation time. She used her magic again, and easily surpassed her sister's boobage. The straps holding them back snapped and sprang free as she grew her girls bigger and bigger. She didn't stop until she could barely see over the top of them. A moment later the last remnant of her bikini top gave up – two little golden triangles lost their grip on her protruding nipples and fell down into the water. “I will do anything it takes.”

That only gave Nightmare a mere moment of pause, though. “You asked for it!” Another flash of her horn, and hers surpassed Celestia's once more, bouncing lewdly as they suddenly increased in size. The nipples alone were the size of a normal pony's fist; it was amazing that she could even stand upright with such a weight on her chest.

Celestia frowned over her own massive cleavage. “You know there is only one way this can end, Nightmare Moon.”

“You wouldn’t dare...”

“You leave me no choice.” Stepping up out of the tub and steeling herself for the unsettling feeling that was to come, Celestia called upon her magic again, but in a different direction. Her crotch bulged outward underneath her little bikini bottom, quickly swelling it full, then overflowing it with a massive white set of cock and balls. Her enormous shaft – easily the length of her arm – spilled out from the top, and her swimsuit, straining at its straps, nearly disappeared into the cleft between her ponderous balls. “Admit defeat, Nightmare, and go back to the oblivion you call home.”

Nightmare Moon rubbed her black hands along the sides of her oversized tits, and her horn began to glow again. “I did not commit thus far to retreat like a coward. Behold!” Nightmare's crotch also bulged outward, and she did not show any of Celestia's self-restraint.

A monolithic black cock sprouted upward and thrust up through Nightmare's cleavage, and massive black testes dropped down to hang between her knees. Their massive girth pressed against her thighs on both sides, making their round contours bulge out forward underneath the huge curves of her breasts. She looked at Celestia, barely able to see over the flared tip of her own black cock. “Look now, and despair!”

A slight trickle of wetness dripped from Celestia's pussy onto the back of her balls and trailed down the smooth curving skin of them. No! She could not submit so easily! No longer wasting any effort on words, she used her magic again. The straps of her bikini bottom snapped, and her expanding balls pressed the thin fabric against the floor as they bounced against it. Her legs spread wide to accommodate their huge, sloshing mass. Her cock sprang upward even more, and she only stopped when its fleshy tip – wide as a serving platter – pressed itself to the spa room's ceiling.

“Enough play,” Nightmare Moon shouted. “We both know how this will end.”

“You don't have to do this, Luna.”

“I am NIGHTMARE MOON!” With a shout that shook the whole palace, Nightmare lit her horn once more, with a glare bright as the sun itself. Her endowment surged out and up, smashing through the stone ceiling. Her balls expanded outward to fill half the room, pressing against the walls on both sides.

Celestia was forced back by the wave of expanding scrotum-flesh, skidding across the floor as she tried to hold her ground against the warm, yielding skin and the inexorable wet heft behind it.

But she was not to be outdone, and she had more of her own to give. She expanded her own inappropriate equipage relentlessly, letting it push back against the massive curve of Nightmare's balls. She was the one who demolished the room, unfortunately.

The walls shattered outward, and out spilled a gargantuan mess of both black and white flesh, each vying against the other, each set of oversized organs fighting a prodigious push-of-war against the other's, even though neither sister could even see each other beyond the veined tree-trunks of their cocks.

By the time they slowed their increases, each was the size of a mountain. Their balls, with their tiny bodies balancing atop, spread out across the countryside, and their stupendous cocks carried their implacable rivalry up into the stratosphere.

Each of them had to spin around, dragging their huge balls across the fields below, in order to get themselves back to back, where they could turn their heads to face off against each other.

Nightmare pointed to the sky. “Behold! My mare meat has eclipsed your precious sun!”

“Don't be so cocky,” Celestia quipped. “You aren't winning this.”

Taking a closer look, Nightmare breathed in a gasp. Celestia's was taller! There! There was a high, wispy cloud Celestia's pulsing tip penetrated, but Nightmare's failed to even reach! “No! I will not stand for it! For the glory of the night!” Calling upon incredible amounts of magical energy, Nightmare surged past Celestia, expanding and expanding. She didn't stop until Her balls outweighed the planet itself. Her vast cock rose to unthinkable heights; the moon, continuing its now-failing orbit, struck the head of her cock and made the whole pillar of flesh bob obscenely back and forth. Celestia was now no more than a speck lodged between Nightmare's balls and the planet Eqqus. “Aha! Victory is mine,” she shouted through the vacuum, down from her lofty perch atop her own balls.

But Celestia did not allow her such an easy victory. Soon, the speck began to grow. A swiftly expanding white cock shot past Nightmare and struck against the side of her own, leaving both of them quivering and shaking, shockwaves reverberating through the rarefied atmosphere. Tremendous white balls grew to match and even outpace hers.

The planet Equus and all its inhabitants were crushed between the two monstrous sets of balls. Neither sister took particular notice of this.

“Yield!” Celestia shouted as the mutual gravity of the titan genitalia crushed the two sisters together. Neither of them could breathe, squashed together by hot flesh as they were. Thankfully, neither needed to.

Nightmare used her magic to begin growing again. “I will never surrender!” When her impossible growth spurt finally ended, all the planets in the solar system had crashed into her balls, and her cock had expanded beyond the reach of their farthest orbits.

But Celestia wasn't about to be outdone, not after this much hardship. She grew as well, absorbing the mass of her precious sun into her swelling balls and bouncing Nightmare half a light-year away with the force of her expansion. Even at that distance, Nightmare Moon was easily visible, though only as a huge cock and balls that eclipsed the stars.

And it was growing even bigger.

“Damn and blast,” Celestia muttered to herself. “Does Nightmare never know when to quit?”

Both sisters kept at it constantly, growing and growing. Their twin pillars of flesh devoured nearby star systems, their balls expanded into interplanetary space ... and still they would not stop.

Finally, swinging a cock the size of one of the galaxy's spiral arms, Celestia turned again to face her enemy.

Nightmare had grown faster! Her huge black cock swung up from below, invisible against the intergalactic void, and smashed into the bottom of Celestia's balls.

Reaching down and cupping her hands against the huge flat surface of them, Celestia cringed. From her tiny vantage point deep in the tiny cleft between her cock and balls, Celestia couldn’t see the curve of them. They appeared like a uniform flat plane stretching out to all sides as her legs did a split across the surface. Her astronomically huge cock blocked another half of her vision.

But she could see that Nightmare had grown even bigger. Impossible!

“Admit defeat, Celestia!” Nightmare's tiny voice came from the void. “You have lost.”

“Not yet I haven't!” Celestia braced herself against the warm, squishy surface of her own balls, and this time when she called upon her magic, she left nothing in reserve.

She grew, leaving Nightmare behind. Even though Nightmare struggled to keep up, it was nothing, nowhere near enough. Celestia grew until she dwarfed her little sister, until Nightmare Moon was a mere black speck against the blackness of the galaxies.

And still Nightmare hadn't given up growing! She was behind, but gaining fast. Something had to be done! ... And Celestia knew just what that was. There was only one way to finish this.

She swung her cock around, swinging its smooth arc through intergalactic space. She winced as whole galactic clusters struck against the skin of it, entire intergalactic civilizations being pulverized by her flesh – they stung a little – but she kept to her purpose. There! Now her almost-infinitely distant tip pointed right at Nightmare Moon.

The force of Celestia's expansion had forced Nightmare away, but gravity was quickly overcoming that. As she grew, Nightmare plummeted toward Celestia's more massive stallionhood. She had turned her body away from Celestia in order to be able to look backward and see, but she wasn't looking now, her full concentration on growing bigger.

With her god-like vision, Celestia could see Nightmare's mistake, Nightmare's vulnerability. She was falling toward Celestia's tip at an appreciable portion of the speed of light. Her legs were spread apart over the surface of her smooth black balls ... leaving her miniscule pussy gaping wide and completely unprotected.

When that little pink pussy behind black lips met the incomprehensibly huge tip of Celestia's white cock, it was the most forceful impact the entire universe had yet seen.

But Celestia had one more spell in her arsenal, one more trick, and it would be Nightmare's undoing. The good old 'yes it WILL fit' spell. She cast it just in time, just before the impact.

Nightmare screamed when it happened, when her pussy lips inexplicably spread over the immense cock head. Almost instantly, she was spread out into a thin film. All of her, even her massive genitals, spread open to accommodate the gargantuan cock. Before she had even taken half of it, her entire being was spread molecule-thin over the surface of Celestia's unbelievable cock.

And yet, more and more of it came! Nightmare had to keep her growth spell at full force just to keep from being torn to shreds.

When the lips of Nightmare's pussy met the base of Celestia's cock – as the fitting spell guaranteed they would – Celestia knew victory would be hers. Nightmare was nothing more than a thin film across the surface of Celestia's cock, barely enough for even Celestia to see. But she could, and she reached down, stroking her fingers over the tiny portion she could reach. “How does it feel to be defeated, Sister?”

A tiny, thin whine came back from the unfathomable distance.

Celestia grinned and started pushing that film up and down over her cock. She could only touch a tiny portion of it, but that was enough. It was working. The pressure in her infinite balls was building.

Even though Nightmare's growth spell was still going in full power, she had barely managed to become two molecules thick when Celestia's immense flare swelled up and stretched her out even more.

Celestia had never felt a tingling rush this big before. “Are you ready?” she asked.

“Noooo!”

Despite the definitive reply, it was too late to hold it back. Celestia came. It was an explosion unrivaled by anything since the big bang itself. Half a universe of cum surged through a tube big enough to comfortably fit galaxies, then shot out of a cock too big to ever be measured.

The thin film of Nightmare Moon stretched some more, then finally burst, spewing tiny black bits of her and galactic swirls of cum across half the universe. And still Celestia came, emptying her cosmic balls, spraying a thick stream of spooge – force enough to annihilate galaxy after galaxy in its path – out to and beyond the edge of the universe.

Plastered to the very front of that tidal wave of destruction was what was left of Nightmare Moon. She was flung so far afield she ended up warping back to the other side of the universe and continuing toward Celestia's back.

Celestia had almost finished when the first wave of hot jizz struck the back of her balls. “So,” she mused, “the universe is spatially cyclical...” An endless stream of hot spunk sprayed against her, covering her balls, her cock, and smothering the rest of her body. She was floating in an incomprehensibly vast ocean of her own semen!

“I... apologize, Sister. I don't know what came over me.”

Celestia used her magic to clear a bubble in the trillion-mile-deep sea of white around her. Within that bubble, Luna fell, yelping. She splashed into the bottom of the small void, then stood on the surface of Celestia's balls, up to her knees in cum as more white stuff slowly dripped off the rest of her body. She was the real Luna now – no more nightmare moon – and she was proportioned just like she always had been. Having every molecule of her body plastered with her sister's ejaculate must have done her ego some good. Her naked belly was torn open from sternum to crotch, showing nothing but more white inside, but this was nothing to be concerned about for an immortal being. She'd be healed up in no time.

Celestia smiled. “Apology accepted. I haven't climaxed like that since... Well, I don't think anyone has climaxed like that in the history of the universe.”

“Indeed.” Luna nodded. “Let us pray that no one ever shall again.”

“Well, at least now I finally get to enjoy my hot tub soak in peace ... once I re-create most of the universe.”

Luna winced. “Sorry about that. Anything I can do to help?”

“Well...” Celestia looked up at her still-hard cock, or at least what little of it she could see within her bubble. “I think I might need another go before I can get rid of this...”

With a devious grin, Luna sloshed her way through the cum until she reached the wall that was the edge of Celestia's cock. “Like this?” She licked it.

A pulse of pleasure thrummed through the universe, and Celestia gasped. “Oh yes... do that again!”

Author's Note

This story is brought to you thanks in part to my Patreon supporters! They're invaluable for keeping me focused and motivated to write with the power of the deadline.

If you'd like to help keep me writing, please check out my Patreon page.