Rainbow leaned back onto the Princess' of Magics bed, relaxing just as she always did under Twilight's ministrations. Those purple lips peppered kisses up and down the tops of Rainbow's petite breasts and neck. Stifling a laugh as Twilight's lengthy hair tickled her, Rainbow gripped Twilight's arm and guided her up to lay alongside her. With a cute sigh of reluctance to pull her lips away from Rainbow's body, Twilight crawled up and collapsed next to and slightly onto Rainbow, mashing her much larger frame down onto Rainbow. Locking half-lidded eyes, Twilight asked through a husky breath, "You never just want to cuddle. What's on your mind?"
Rainbow grinned and closed her eyes for a moment, just enjoying Twilight's much larger chest mashing into her petite body. "Hmm... Do you remember that pervy sex book you told me about?"
Twilight smiled, and nodded enthusiastically, "Oh how could I forget? There were so many interesting spells I've wanted to try! What, were you thinking about spicing things up tonight?"
"Maybe it'd be good to give your tongue a rest, and we could try something more traditional. Gimme a dick so I can ream you!"
Twilight couldn't stop smiling, and rolled over on top of Dash, straddling her slim hips and pressing her larger alicorn body down onto her. "That's not a bad idea, but I've never tested the spell. It goes against my morals to let anyone but myself be the guinea pig for first time spellcasts." Twilight's horn lit up and Dash's wrists and ankles glowed violet. Without giving Dash a moment to protest, she pressed her lips against Rainbow's and spread her limbs apart slowly, spreading her legs and pinning her arms flat against the bed.
Rainbow Dash grunted weakly, her plans unraveling, but it wasn't so bad, Her limbs splayed slowly across the sheets and Rainbow was made fully exposed and helpless under Twilight's powerful magic. The kiss came to an end and Twilight sat back onto her heels, her horn glowing further as she conjured up a second spell. Twilight spread her knees and her swollen clit lit up with her magic as well, and she groaned in concentration and pleasure. Rainbow simply relaxed into her magical restraints and watched the spells fly, taking special note of the subtly spreading purple pussy on display. After a short pause Twilight's clit began to swell further, lengthening and jutting out. Centimeters were added on, and quickly turned into inches as her clit was shaped into an equine penis. Her moans escalated during the transformation, clamping her eyes shut as she desperately applied the final touches to the spell.
"Holy shit Twi, I said a cock not a weapon of mass destruction." Rainbow shifted nervously as Twilight's member continued to grow, halfway between flaccid and erect and already approaching 14 inches. Twilight only arched her back and continued feeding the spell, swelling to an unnatural length and thickness. Rainbow's surprise turned into laughter, "Alright Twi very funny, you couldn't fit that in Rarity anymore. Quit messing around and-" Twilight's magic surged once more and Dash's lips were sealed shut in its aura. The moaning died down and the transformation spell finished, rounding out the magical phallus to just under two feet, and as thick as Rainbow's thigh.
After a gasping breath, Twilight finally spoke up, "Oh I'm not messing around. I can DEFINITELY make this fit, just you wait and see." As she spoke she leaned in and pressed the massive tip of her cock over the entirety of Rainbow's crotch, pressing into both thighs in her attempt to reach her pussy. Dash grunted a few times, squirming in her magical bindings to draw her hips away from the monstrous shaft.