celestia experiments with a dimension spell. every time she casts it, it takes her to a universe where Luna is bigger than the last. And Luna's in heat.

Celestia slowly opened her eyes, feeling the effects of the magic dissipate around her. Normally, she wouldn’t be one to test out experimental magic, but she’d dug it out of an old drawer after the New Year and couldn’t really think of a reason against giving it another go…

“So?” Luna asked. “Did it work?”

Celestia stared. She seemed to be standing in the exact same place she’d been in before: the middle of her bedroom, with Luna looking at her expectantly from the bed.

“It…would seem not,” Celestia said. “Odd.”

Luna dropped to the floor and shrugged. “A pity. I was wondering if you might find some use for that old magic…or at least some entertainment.” Her cheeks flushed red, and Celestia realized that Luna hadn’t had any…entertainment of her own since she’d returned from the Moon. Oh, no. She couldn’t be in heat…

Wait. Something was off. It took a moment for Celestia’s mind to catch up to speed, but it wasn’t long before she managed to put her hoof on it: Where before Luna had been near a head shorter than her, the two Princesses now looked eye-to-eye.

“Luna,” she said. “Did you just use some manner of shape-shifting spell? You look taller.”

Luna looked down at herself. “This is the same height I’ve always been, Tia. Why do you ask?”

“I…” So it had worked, then. Celestia inwardly congratulated herself. Then again…it was somewhat unnerving to be in a universe where Luna could look her in the eye. “Let’s see if it still works,” she said with a forced smile. “Just one more time.” And hopefully back to her home dimension.

Poof.

“One…more time? I…thought it didn’t work?”

Celestia blinked. She was still in her room, but Luna no longer stood in front of her. Now, instead, a mare nearly twice Celestia’s size lay back on her bed, her long legs draped over the sides.

Celestia’s cheeks flushed red even as her eyes widened. Luna had gotten even bigger - and her hoof was placed right over her nethers.

“Mmff…” the enormous Luna moaned, just under her breath. There was a sizable sag in Celestia’s mattress, just under where Luna had planted her massive flank. “Any…success?”

Celestia stared at Luna’s winking clit, its folds almost the size of her own Cutie Mark. “It…would seem so,” she said, mildly disturbed. This was getting strange - it was past time to return to her home dimension.

Poof.

Celestia had hoped to find herself back in her bedroom with a normal-sized, controllably horny Luna. But when she opened her eyes next, and found herself staring down a dripping pussy the size of her head and a room entirely full of Luna-bulk, she found she could only groan.