You watch surprisedly as your girlfriend's body starts to change in a very odd way. Her skin seems to discolour to an odd greyish purple shade, and as she undresses, you see her nipples turn a similar colour to fit. Her hair shifts to a vivid purple, unnatural for humans, and as she blinks a few times her eyes become purple too. Her lips deepen in colour until they're also dark purple, and you hear a slight crackle as her teeth sharpen in her mouth. She pants, as if pushing on something inside her, until you see the biggest change to her body- a focusing crystal forms on her forehead, a deep red organ that glows and seems to sense you.
In particular, it senses that you're aroused, and she walks up to you with a teasing grin. "You recognize this form, don't you?" she asks.
"Fuck," you breathe. "You're a Rymian huntress."
"Yep," she purrs, bearing her teeth like she's threatening to eat you. "I've seen you fantasizing about mating with one of my kind, so I thought you might like the opportunity to do so for real." You nod. "There's just... one small caveat. If I were to mate with a human, our offspring would prove infertile. So, would you like to take on a different form for mating?"
You can't imagine a hotter form to mate with, but you're not sure how to make your form as appealing to her...