Your girlfriend decides to change her name to Safu. As she informs you of this change, her body begins to tremble, as if in constant subtle orgasms. It is as if she has always been Safu and admitting it is bliss, pleasure, and endless potential.
Safu relaxes, and as she does, her clothes begin to tight. Her breasts begin to swell larger. Her hips begin to broaden and thicken fuller, her asscheeks plumping with fuckable fat. A huge, white-furred sheath lurches from her lap and a massive pair of white-furred balls grow to cover her thighs. She's transforming into a hyper-herm mouse!
"S-Safu?" you gasp. "What's happening? What's going on?"
"Oh, you noticed!" she laughs, her voice deepening, turning masculine and husky. "Don't worry about it, honey. It's just a bit of a change, that's all."
"Change? Changing to what?"
Safu grins at you. She's not the girl you know. Her body grows larger as she pumps through a new puberty in seconds. Her muscles bulk and harden, her hands and feet turn into giant paws, her fur covers her skin, and her tail begins to wag. Her clothes rip away, leaving her naked, but she doesn't seem to care. The last thing to grow is her cock, a massive pink shaft with a pointed tip, covered in bulging veins. Her balls are as big as your head.
"I'm turning into the ultimate hermaphrodite, honey. An incubus! A mouse demon with the power to corrupt others. I can do anything to anyone, and you're going to enjoy every moment of it." 
With a girlish giggle, Safu turns to where your roommate is sleeping. As you watch, she opens their door and pulls you inside to watch her work her magic. The boy begins to moan in his sleep, and then to squirm. He is being pleasured, somehow, by Safu. He wakes up, staring at her in confusion.
"Good morning, cutie," says Safu. Her voice makes the boy's underwear tent hard. His face becomes flush and he stammers and gasps in surprise.
"Sa...Safu?" he moans.
"Oh, no, no, no, cutie," she says, reaching down and squeezing his bulge. "My name is Mouse, now."
"Mouse!" the boy groans, his hips lifting from the bed and pushing into her grasp. "Ohhhhh! M-mouse! You look amazing!"
"Aw, thank you," giggles Mouse. She begins to rub the boy's cock through his underwear. "Now, be a good boy for me. Get pregnant. Get pregnant right fucking now and give birth."
The boy cries out with ecstasy, his cock spurting cum, his stomach suddenly swelling, his cock growing, his chest filling out with huge, heavy breasts. You stare in shock, barely able to comprehend the transformation. As the boy's cock hardens again, Mouse's demonic powers are unleashed. You hear his mind break and the boy is suddenly smiling in adoration and awe, his cock dripping precum onto his belly.
"W-what's your name?" asks Mouse.
"L-Lucas," moans the hermaphrodite. "Y-you did this to me, mistress? Ohhhh, it's incredible. Thank you."
"You're going to be such a cute mother," she says, leaning over and sucking his fat, pink nipple. "Oh, fuck, I love that taste. It's sweet as honey. Here, have some."
She leans across the bed and shoves her breast into your mouth. Her nipple spurts sweet milk onto your tongue.
"Oh, no," you cry. "You're not going to turn me into one of those...