"Ah, what a fine specimen you are!" said the tall, pale, pointy-eared figure in front of you. "Or at least, you're sturdy enough to become one."
"What do you want with me?" you ask, in awe of the torch-lit stone hall you've found yourself in. "How did I even get here?"
"I want you, but not in your present form, of course. You've been given a great gift tonight, human. The gift of a noble elfen master to serve and millenia to spend in service. Much more dignified and worthy than whatever it is your frail-bodied brethren waste their paltry decades on these days."
"Everyone knows there's no such thing as elves, and I will NOT be your servant!"
"Everyone knows? What silly notions are you humans teaching each other now? Regardless, you most definitely will not serve me in that absurd body of yours: so elf-like, but so insultingly fragile! A proper elfen hunting beast should be stronger and, frankly, more bestial."
"Let me go," you spit, "and get me some clothes!"
"Clothes? On a hunting dog? Even for a mighty fae hound, that is presumptuous. Your ambitious pride will serve me well in the field, in a century or so, when you're fully trained."
"I ain't no blasted dog, you pointy eared freak! Wait, what's this? What's going on?"
Hairs are sprouting from chest, and from your arms, but they are not your normal color. As the hairs grow longer, continuing to sprout from new places, you can see that they are GREEN hairs, growing longer, thicker, shaggier almost everywhere they have skin to sprout from. A growing strength courses through your hairier form, which must be bulging out with muscle under all of that green hair. You're bulking up, flexing your arms, feeling their strength.
"You like your newfound power, hound? You will see many glorious hunts in your new form."
New and thicker bones, skin and muscles keep swelling from within you, until you are taller and wider than the elf, like a hairy ogre. Green hair is draped from all of it as far as you can see around yourself. Your ears swell, and you feel the stretch far and wide, improving your hearing, until you can detect scurrying animals and rustling trees far in the distance, beyond the castle. Then you wince as jade daggers pierce themselves out from every finger and then every toe. You gaze at the massive claws in terror, fearful of hurting yourself, soon beginning to wonder whether you could fight your way out of this with such a powerful new advantage.
Before you can think of how to use your clawed hands, you fall over and must hold yourself up with them. Even now, on all fours, you are at least as tall as the elf. Something throbs and twitches near your bottom, so you snap your head back to stare down to the end of your quadruped body, but you can't really see much of anything under your green fur. A pressure builds, and a shaggy green mass spills out to hang between your legs upon release. At first you think it's just a bunch of your green hair, like a woman's pony-tail, but then it rises from between your legs. You see and feel it move behind you, and you know you've got a tail. You force it between your legs in hopes that the elf won't see it, but then you feel a pressure in your nose. It stretches, fusing with your mouth to form a muzzle, taking in scents from all around you, but most interestingly the scent of an intense electric tang around the elf: magic, you somehow instinctively understand.
"Ah, a proper elfen hound at last!" says the elf with a few claps. "You will soon be the perfect companion for me in tracking down more humans in your realm, bringing them back here so I can transform them, and even just hunting other magical beasts for fun and profit. Now, let's get down to business!"