"I-I don't know..." you stammer for a moment. "It's such a big decision... I mean I want to but-"
The headmistress holds up a hand. "I understand, but you should look at this as an opportunity. Being raised human means you'll have a significant advantage in blending into human society once you eventually manage to take human form. In addition becoming a mythical creature will grant you greater abilities than any human on the planet along with an extended lifespan."
You nod. What she's saying makes sense, and the school was one with an exemplary record. This could easily be a massive advantage to you. "Ok, so going by that line of thinking... What gets me the most of those two things?"
The headmistress taps her finger against her lip. "A Succubus I should think."
You're quiet for a second. "...Sorry what?"
"Yes... there are more powerful creatures but they don't live as long, that the more long lived creatures aren't as powerful." The headmistress produces a book seemingly out of nowhere and opens it up so you can see it. Inside are black and white line drawings of several beautiful women with horns, tails, and wings.
"But aren't they sex demons? More importantly aren't they GIRL sex demons?"
"They do have infernal heritage yes, though you would not be required to reside in hell or even interact with other demonic entities should you choose otherwise. As for their sexual nature they do have a heightened sex drive but that is also true of many mythical creatures. You'll find ways to manage." She snapped the book shut and it disappeared just as easily at it had appeared.
"...I think you're skipping over the girl part."
The headmistress smiles and places a hand on your shoulder. "I understand. Yes, all succubi are female and no I can not turn you into an incubus for... various reasons. I'll understand if you want to-"
"No," you say and you're a little surprised that you say it. "It just... it caught me off guard is all." You look at her and nod, "Do it, make me a succubus." You're slightly surprised by the sexual tingle of arousal that passes through you at those words.
The headmistress raises a hand and says "Deep breath," before there's suddenly a flash. You feel something twist inside of you before a surprisingly pleasant warmth seems to suffuse your entire body. As you look down you see yourself get a bit shorter overall while your arms and legs become thinner. Also becoming thinner is your waist, pinching in while you can also see the beginnings of breasts pushing out your shirt. The feeling of your ass filling out is making your pants uncomfortable and the accompanying feelings in your hips are not helping matters. Your hair turns black and grows to fall down your back in a flawless silky curtain. Strange sensations from your face also tell you that changes are occurring there. Most likely you're becoming more feminine looking.
Other more exotic things are also happening. Your skin is taking on a red colour slowly but surely, a bright tomato red. At the same time you can see your fingernail's lengthen and turn jet black. A sudden shooting pain from your rear makes you jump and let out a feminine squeal as a spaded tail of the same colour as your skin bursts out of the back of your jeans. You try to twist around to get a look at it but are distracted when your t-shirt suddenly explodes when two black bat like wings burst from your back. You gasp at the suddenly low temperature of the room and look down at your suddenly exposed chest. It's of course the same red as the rest of your body but your nipples are pitch black.
The final sensation is two pinpricks on your forehead and as you raise your hands up to them you can feel two small horns force their way out of your forehead, near the hairline. The changes come to a stop and you're left standing there in the middle of the office, eyes wide and staring. The headmistress wordlessly produces a mirror for you check yourself out in.
You see as sexy demoness with cherry red skin, busty, topless and wearing a pair of torn jeans. Your lips have turned the same black as your nipples and fingernails. You wonder what other parts of you might be the same colour but before you can check you spot your tail and flick it around experimentally.