"So, what is my trial in this alternate world?" you ask the unconscious voice.
"Well, I'll give you a clue with a vision," it says teasingly. You see someone's conception- a bunch of sperm surround an ovum, and one pushes inside. But then the vision rewinds, the sperm that got inside pulls out, and another one pushes inside instead.
"I don't get it..." you admit.
"Every being that has ever existed has had two ways they could develop, and there's about an even chance of it being either way," the voice explains. "In this world, you get to develop the opposite way to how you did."
"Wait, so do you mean-" you start, and then feel a weird twinge race through you.
"That's right!" the voice says, with a teasing giggle. "In the conscious world, you were male- in the unconscious world, you'll be female!"
You watch helplessly as the void's power slowly emasculates you. Your figure reshapes itself, turning more slim and curvy while your shoulders slink inwards and your hips flare outwards. You notice your voice has softened and turned higher pitched. Your arms and legs are less muscular and more lithe. Your skin is smoother and most of your body hair has vanished.
"Now for the fun part!" the voice says chirpily, and you bite your lip as a warmth runs through your chest and your nipples perk up. They feel weirdly sensitive and seem to be swelling to a weirdly large size. "As a female, you'll need to be able to feed your young, so you've got to grow breasts. Not to mention it'll feel so nice having people play with your chest and suck on your nipples!" Somehow your body accepts what the voice says, and you sprout boobs rapidly, the soft mounds growing larger and larger and more sensitive. Not only that, you're starting to fantasize about how good it'd feel to have someone cup them, or with your nipples.
Now your breasts have developed, the intense warmth moves further down, and you realize you've got an erection. "Sorry, you aren't going to have a penis anymore," the voice purrs. "That's for males, and you're becoming a female. But trust me, having a vagina feels waaaaay better."
A part of you wants to object as you see your manhood slowly shrink away, but as it does, the nerve endings seem to bind up tighter and tighter together as the shaft gradually becomes a nub. "See?" the voice reassures you. "You don't need to fuck people with a penis when you can get them to rub your clitoris until your head goes blank with pleasure."
A shiver runs through your scrotum as your testicles pinch up, and with a pair of wet pops, they move up inside your body, and the sac becomes almost flush with your crotch. You're amazed how good your castration felt. "Good girl," the voice says. "I'm glad you see you don't have to be a boy anymore. Your body will be perfect for them to fill with their virile sperm and mate you." You know it's right- it feels odd to think you ever were male, it feels so right to be female.
To finish your transformation, you feel a womb and vaginal tract form inside you, and the sealed skin between your legs sifts as it extends towards your flesh. "You're seconds away from womanhood," the voice reassures you. "In the womb your labia were fused to form a scrotum- pry them apart again so that you can have your vagina filled with warm male seed!" You do as it asks, and feel your slit peel open, female juices dripping from it.
You look over your body in astonishment, seeing it's been wholly feminized. You're amazed by how there wasn't a hint you'd ever been male, and by how intense your emergent feminine sexuality felt...