Her legs multiplied, leaving her with 8 legs before they got thinner and longer. A sphere grew on her ass, which darkened along with her legs until they were completely black and shiny with some red lines. The bottom half of it was totally the body of a huge spider, but the transformation ended only after her eyes went full, between red and a second pair opened just above them.
Totally surprised you said "You're a...."
"...A drider. Exactly." she completed confirming before continuing "I believe you don't know but I am not the biological daughter of the couple who own this house. They adopted me after they found me alone in the forest when I was still a small child and adopted me. I was lucky at that time, because I was alone weeks after my mother was killed by people who claim to be monster hunters, she had sacrificed herself so I could escape. We driders can temporarily assume human form, so I was able to hide my true form, so much so that nobody in my adoptive family suspects that I am not human."
"I've heard about driders and knew they eat humans. You mean you only approached me so you could eat me?" you said
"Don't be silly, this story is totally false. We have been dating for three years, if that were true and I wanted to eat you I would have done it a long time ago and would never show my true form like that. But it is worth remembering that for me to reveal my secret is a matter of life and death and I only revealed my secret to you because I trust you. I'll understand if you don't want to date me after this." your girlfriend said
You....