In the halls of the Castle of Friendship a unicorn and and an alicorn were gathered in the library, Twilight was standing at the blackboard with a list of of spells prepared and detailed, but her student was not her usual student. As instead of Starlight glimmer at the table opposite her it was Rarity instead, looking somewhat confused. As if she was wondering whether or not she should be here or not.

"Twilight I'm not so sure if this is necessary. I'm just a simple fashionista darling, I hardly think I’ll need to know about extra dimensional magic dear.” Rarity pointed out, her lips pursed.

"But it would be easy for you to master it. With the Root of Magic you're greatest prodigy in history. Imagine how powerful you could be." Twilight pointed out, all but hopping with joy. Much to Rarity's awkwardness it caused Twilight's now enormous breasts to jiggle in such an enticing way that Rarity couldn't help but be distracted by.

"Rarity?" Twilight asked sharply, bringing Rarity back to reality.

“Sorry Twilight what were you saying?"

Twilight smiled, clamping her hands on her knees. “I was just saying how you could use your power to impress Spike. Imagine what you could do for him now.”

Rarity froze like a deer in headlights, her creative mind was a whirlwind of possibilities of how she could use her power for Spike’s benefit. As a friend certainly she could find all sorts of gems and over gifts for her dear Spikey Wikey. As a lover she could make her dates with him truly magical and the sex even more so.

And then there was what she could do as a sex slave…

She could fashion erotic designs that teased and tantalised. She could reform the bodies of herself and others, giving massive proportions that would make the wildest of teenage fantasies seem mild. For her fashionista mind the body could be made an extension of her own talents and one that her beloved Master could happily drool over.

Or perhaps she could use the power to recruit for him, her naughty depraved mind thought to her friends. Of hypnotising them into perfect sex slaves or breaking them Spike's certainly magnificent cock. Of using her magic to bring even the Princesses into his harem, effortlessly overpowering the Goddesses with their own magic and offering them as sex slaves to her Master Spike. She could even go one step further and use her overwhelming power to impress and inspire the most beautiful, intelligent and sexy mares in all of Equestria to become the eager sex slaves of her beloved Spike.

And she would be the best of them, the best sex slave in the harem, no the world. The one more beautiful and sexy than models, more desirable than the Princesses, more loved than her friends. The diamond in the horde. It would be paradise. It would be-

She was drooling, right in front of Twilight. Who was smiling with that annoying ever so knowing smile that caused Rarity to turn scarlet. "You were thinking about your fantasy again."

Rarity knew she could share these sort of secrets with Twilight, she had said so. And as the Princess of Friendship Twilight had the final say on which boundaries were too far, she knew she could trust her.

“Was I really that obvious?” Rarity whispered, her face a rosy blush.

"Oh absolutely, but it's nothing to be ashamed of." Twilight assured her with a smile. “In fact why don’t I show you some spells I’d think he’d like.”

Rarity's eyes shone with gentle curiosity. "Are you sure Twilight? I mean I don't that would be wise Twilight. This fantasy of mine isn't something that is all that morally correct and I don't think it would be wise to tempt me with that power."

Twilight laughed, on the surface it appeared to be a good natured laugh at her insecurity, but deeper inside it was in fact a laugh at the supposed irony of Rarity's statement. But she soon found herself pulling Rarity into a friendly reassuring hug. "You don't need to worry Rarity. You're my friend and I trust you to use this power for the right reasons.”

Rarity let out a large sigh of relief, thankful for her friend's support. Perhaps Twilight was right, perhaps she was being overdramatic. It certainly wouldn't be the first time. “Thank you Twilight.”

“And personally I wouldn’t mind it so much if you used this power to create a sex harem for Spike, even if it's on me." Twilight whispered with a sneaky grin. In an instant Rarity playfully pushed Twilight off of her.

"Twilight." She whined, shoving her friend off playfully. “Don’t tempt me like that.”

“Why not? What would be the harm?” Twilight asked, not so subtly swaying her breasts in a way to entice and distract Rarity.

"It's immoral and wrong Twilight. Can you imagine what a noble drake like Spike would think of me?"

"Yeah, you want to turn all the most of beautiful women in Equestria into Spike's personal sex slaves, including yourself. I'm sure he'd be horrified." Twilight said, her voice dripping with sarcasm. Something that Rarity couldn't help but take offense to.

"For your information Twilight Spike is a noble, gentle dragon who treats women with respect. Not some disgusting pervert." Rarity yelled in defence of her loved one. Frustratingly Twilight simply rolled her eyes.

"Yeah but he's still a guy. I guy who wouldn't mind being surrounded by a harem of sexy mares." Twilight pointed out. "And with this spell you could make a whole lot more of them."

Rarity backed away. "Twilight what are you-"

Twilight's horn lit up and with a flash Rarity's body changed. Her clothes were ripped apart, with the exception of her lingerie underwear, her hair and make changed until it wouldn't be out of place on the body of a hooker. But most extreme of all was her body changed into an hourglass shape so extreme that she couldn't imagine that all the organs in her body should be able to fit were it not for magic. Her breasts were absurdly huge, the breasts only half covered by her bra and still threatened to spill over them whilst her ass ballooned outward.

"TWILIGHT!" She squealed, as her hands explored her body. Trying to get a handle on just how much Twilight had wrapped and twisted her body. "What have you done?"

"Too much?" Twilight asked with an innocent little shrug.

"A little." Rarity snarked back.

"Then just fix it yourself." Twilight suggested, nearly causing Rarity to snap at her in anger until she realised that with the Root of Magic's power she could indeed just do that. She had seen the spell and yet again had done what Twilight took years to master in an instant. More than that she instinctively now knew that she could cast this spell on anpony in Ponyville and far better than Twilight could, even creating cross species hybrids or changes.

Twilight was smiling with a grin. "Or would you like to try it out first?"

Author's Note

So currently the poll has Pinkie Pie leading at 61, followed by Zecora at 52 and Mrs Cake at 49. It was neck in neck for a while there but whilst there is a possibility for an upset it seems most likely that Pinkie Pie has won. So suggestions as for what part she could play are welcome as always.

Straw poll here: https://strawpoll.com/polls/05Zd1oQlby6/results

On another note H0rs3Fly at Fimfiction has raised a good point as to what the story should do for candidates like Mrs Cake who are married. While the plan for Shining Armor is to genderswap him and bring him into the harem (having been made to have an open relationship in the meanwhile), I don't want to do that with anypony else. As I see it there are ten realistic options:

1. No cheating, all married women are just divorced in this reality
2. Things are magically changed so the husbands have been divorced for a while.
3. Things are magically changed so that their husbands have a mutually open relationship.
4. Things are magically changed so they're okay being cheated on but keep married
5. Things are magically change so they're kept blissfully ignorant that they're wives have become sex slaves
6. Things are magically changed so they're okay being cheated on and happily divorce their wives to become sex slaves to Spike's harem
7. Nothing is magically changed, but in this reality the husbands are assholes for whatever reason who deserve to be cheated on.
8. Forgetthe husbands do what we want
9. Don't target married women
10. None of the above

When I say magically changed I mean with either the Root of Honesty, or any other methods the harem comes across. If not then it's just because of the alternate universe this story takes place in. In order to come up with a solution I'm creating a straw poll.

It will allow for multiple options so don't choose your favourite, choose ALL the ones you're comfortable with. This isn't a vote it's a survey so I'm not following a majority here, just trying to get an understanding.

Link Here: https://strawpoll.com/polls/QrgeVN6VRZp