"... A demon" she answers.
"Holy crap! But can you control yourself or anything?"
"No, the witch decided to put a demon inside me which wasn't too powerful so it only appears at Halloween's full moon, the thing is that the demon is a separate entity of me and I have no control over that, I even forget what happens after I transform until I turn back"
"That's actually scary sweetie..." You hugged her, "that can be really dangerous, there has to be a solution to this!"
"I wish, but I don't know what the demon is capable of and what does she do with my body, the only thing I know is that is called Nija"
"And what it will take for you to transform into her?" She checked outside the window, for her surprise the shiny full moon was revealed in the sky. "Now..." She gasped.
You saw how your girlfriend's body started to levitate and change, growing to almost 7 feet ripping all her clothes exposing her giant naked body, her skin started to turn purple as her hair painted white growing two long back horns on top of her head, her breasts inflated from her B cup pair to F cup ones, replacing her nipples with mouths, her legs covered in black fur, changing her feet into hooves and shaped themselves like a pair of horse legs, at the top of her butt a tail grew with an spade tip at the end. Her transformation was finished, she wasn't your girlfriend anymore but that thing called Nija.
"Hmmm" she started to get used to her body, squishing her breasts. She turned around and noticed you, "Oh~ it seems that I'm not alone this time"
"W-what are you! And what have you done to her?" You asked in fear.
"I've done nothing to her, darling~ I unfortunately live in this body, you must be her nice and cute boyfriend".
You nodded. "But what do you want from her, why can't you leave us alone"
"You see, I was a powerful demon back before in the underworld but I was expelled to this boring world with no powers, I was a mere spirit wandering which none could see, until I met my friend Morgana the witch." She levitated to the window staring at the full moon, "She had the ability to transform me back into my prime form so she put me inside the body of this ignorant girl and tried to help me in this form, sadly I have only a mere fraction of my powers and that's why I only appear in Halloween's full moon"
"I can't wait for this night to end..."
She quickly approached you, "Oh no darling~ now that you're here, you'll help me with this~" she wrapped you with her tail, sweating thinking on what she will ask to you.