Ah, that's quite the whimsical scenario! If we were to imagine the dialogue for this surreal crossover scene, it might go something like this:
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Magolor* (laughs deviously) Oh, what have we here? A delectable blueberry pie, just waiting to be enjoyed? Don't mind if I do!
Mettaton* Oh YES! Darling, that's no ordinary pie, that's a special prop for my show!
Magolor* (takes a bite with glee) Prop? My dear metal showman, good food should never be a mere backdrop! Mmm, spectacularly scrumptious!
Mettaton* I'll have you know, that pie was engineered by Dr. Alphys. You'll regret that impulsive appetite—just you wait, darling!
Magolor* (chuckles, then slowly realization dawns) Hm? What's this peculiar feeling? I feel... taut.
Mettaton* (raises an eyebrow, amused) Feeling a little... full of yourself, are we?
Magolor* (panic sets in) Wha—Ack! I... I'm turning blue! Blue as the deep eternal cosmos!
Mettaton* It seems the pie was filled with a little something extra—a berry convincing transformation agent, one might say!
Magolor* (his body starts inflating) No, no, no! This isn't how Magolor envisioned his victory! I'm puffing up like some kind of... Blueberry!
Mettaton* (starting to enjoy the spectacle) Spectacular! I couldn't have asked for a better act! You've really inflated the ratings, as well as yourself!
Magolor* (now fully inflated, floating helplessly) I...I demand to be returned to normal! A master of dimensions, a ruler of rifts should not be subject to such spherical humiliation!
Mettaton* Don't worry, my spherical friend. Once the effect wears off, you'll return to your regular villainous self. Until then, enjoy being the berry center of attention!
--
And there we have it—Magolor ends up as the star of his own comedic misadventure.