Celestia probably should have read through the entire description of the odd spell before casting it, but she doesn't see how there could be any danger. Celestia has encountered magic like this before, but never any that she couldn't reverse easily. Most transformation spells are too weak to affect an alicorn at all, let alone trap one against their will. So she doesn't think anything of letting the magic take hold and begin affecting her, curious about what it feels like.

It's rather pleasant actually! Quite tingly, with a delightful relaxing quality. It appears to be turning het into some sort of inflatable pool toy, though there's something off about it that she can't quite place.

After a few minutes though, she decides she's had her fun, and she should reverse it before it finishes. She attempts to light her horn, anticipating a brief burst of powerful alicorn magic that will shred the spell as it has many times before. Which is why she is startled, when instead of stopping, the process speeds up. She concentrates, but quickly realizes that she can't use her horn at all. It has been transformed into a simple inflated tube, now useless for casting magic, and any attempt to do so feeds power to the spell now spreading across her form.

The princes wobbles, footing becoming unsure as her hooves fill with air. With the progression of the change up her limbs, a strange stiffening occurs, as her new skin settles into it's pre-determined shape. Her front legs buckle under her weight, and then freeze into place with a series of squeaks as she is forced into a kneeling, bow like pose.

Worried now, she tries to chant a verbal spell, only to find her mouth is stiff and hard to move. An unseen pressure seems to press at the inside, massaging her lips, teeth, and tongue into a new shape, before moving down her throat. She squeaks in affront as a similar sensation develops... elsewhere... and she comes to the conclusion that the spell isn't turning her into a pool toy after all. Sex dolls are a mostly foreign concept in Equestria, but she has visited other dimensions a few times, and she recognizes what she is becoming.

The pose does make more sense now.

As her flowing mane and tail transform into stationary balloons, she contemplates her options. It's not like the spell is anywhere near strong enough to hold her forever. ... Right?

Oh. Oh, she may be in trouble. If this had been cast on a regular pony, it would have run out of magic eventually. But with her much larger magical potential, it could last indefinitely! She might be stuck here until someone comes to check this corner of the archives.

Well, that shouldn't be too bad. Luna does know she's down here, and Twilight visits this library often. She'll be down here for a few hours, a few days at most, before someone can locate her and cast a counter spell. And it is so relaxing... She really does need a vacation. Maybe she should just stop worrying and enjoy it.

So she stops struggling, and leans in to the sensation as the final changes occur. Her new silicone holes freeze into shape, her wings morph into simple shapes at her sides, and the transformation closes in on her face. In the last few moments she is able to move her eyes before they become painted silicone, she glances back down at the book that the spell had come from.

There, on the page, a bit of red ink catches her eye.

WARNING: CONTAGIOUS!!! SPREADS THROUGH MAGICAL OR PHYSICAL CONTACT!!!

Her last remaining eye flies open, before the spell reaches it at last, and forces it into the lazy, half-lidded expression of the doll. With a final shudder, the new toy comes to a rest, magical glow fading from the air. And there it will wait, until a new victim stumbles into it's trap.

I've almost got another animation done, and I've been working on other commissions in between them, but work was rough this week and I really wanted a break to do something personal. I do have a half-finished Elements of Bimbo Pinkie that I've been working on, but I wasn't in the mood. I've wanted to do some pool toy/blowup doll tfs for ages, so I went for it. I also have a Luna sequence sketched up. That one makes it much more... obvious, exactly what kind of doll the transformation has made her into. The pose on this one didn't leave that all that clear.