You hear a pair of chilling snaps and quickly turn around to find Sarina standing upright, and completely unconscious. It is then you notice her knees have completely inverted, now bending her legs backwards instead of forwards. She is wobbling so you quickly attach a web to the back of her neck and then to the top of the cave in order to keep her standing upright through her unconscious state.
Seeing her legs bent like this pretty much answers why she fell unconscious. The pain of shattering both her kneecaps, and then regrowing them on the wrong side must have been far too much for her mind to handle. You feel bad for her having to go through something like this, and cannot help but feel somehow responsible, even though you have no idea as to how you are.
But you do keep a few eyes glued to her as you go about your business, just incase she needs you. You watch as over the course of a half hour, her feet break over and over again until they have split into 3 segments each, with a fourth segment pointing in the opposite direction of the other 3 on each foot.
Another alteration is that her pussy has merged with her asshole and now points almost directly backwards and just slightly downwards, unlike usual. It also has widened and grown larger inside. You are quite relieved by this, as it means laying your eggs won't hurt her nearly as much anymore, if at all now.
It is then that her entire lower torso, from her crotch to just below her boobs, explodes out in a plethora of red colored scales, not unlike a snake's. You can guess what is going on now, and while it is unfortunate that she isn't an arachne like yourself, you also understand that it never would have worked if she was to become one. As you are the only arachne in this cave, and you do not want to share it with any other member of your species instinctivly. If she had become like you, you both would have fought for the cave eventually and would have eventually either kicked the other out or killed them. So this was kind of a good thing in the end. At least, that is how you choose to take it as being.
It is over 9 hours later when Sarina finally comes too. For obvious reasons, she is feeling unbalanced and completely not herself. You are there however and comfort her almost instantly, using your body to block her view of her own for a few more seconds as you hug her. You keep her in the embrace while you describe what happened to her first, before she can see it for herself.
"Sarina, you must remember that no matter what, unless you turn into an arachne, I still will love you, okay. So even now, I love you and hope you do not leave me. But I must warn you, you are not the same anymore. You, have undergone a rather drastic change, as I am certain you are feeling about now" you say as calmly as possible.
You then release her and bring her the mirror. "Just, try and not focus too hard with your eyes for now. And don't look at me quite yet. I have blindfolded you because of this. Just do not look at me, okay? Promise me this, and I will untie you."
"Aria, what is going on?"
"Say it first. I need to hear you say the words. I will explain everything, but you must promise to not look at me, not even a glance, until I say it is okay to do so."
"Okay, I promise, now can you unbind me?"
"Say it all!"
"I promise not to look at you until you say I can. Is this okay now?" Says Sarina now rather frustrated with your insistence on this.
You sigh, and get behind her back. You slowly fix her head in place by binding the sides of it to the cave walls and ceiling with webs. Now she can only look forwards, up and down, and slightly to her sides, but never behind her. You then snip the web blindfold from her eyes and run behind a bend in the cave, ensuring you are not able to be seen even if she somehow managed to turn around.
After 3 seconds, you hear a loud scream and "What the fuck happened to my legs and body!?"
"You became like this over the last 9ish hours. You are half cockatrice now. I don't know if you can turn things to stone though, thus the blindfold. I put a spider in the room with you. Look at it and tell me if it turns to stone." You shout from around the corner.
"I-it's fine. It isn't turning to stone. And I am looking right into its eyes too. You should be fine. I understand now why you were being so cautious. But how did this happen? Is it because I laid eggs inside a monster's nest? I'm not saying that that you are a monster, well technically you are one, but I didn't mean it like that. I just meant that... can you untie me please?"
You untie her and then unrestrain her head. You also free the sacrificial spider who runs away quickly. You then turn and face Sarina. You can't help but look at her weirdly shapped form. Human sized chicken legs beneath what essentially is a massive snake's torso. Her normal body is still fully intact from her navel upwards though, save for her new red comb parting her hair, and the way she has her arms folded back against her sides, almost as if they were instead a pair of wings. They clearly are still her arms and hands, though how much of their previous movement capabilities she has retained is still unknown.
Sarina then suddenly squats down and moans as she pushes quite a few eggs out of her cloaca, "Oh fuck! Aria this feels.... Aaaahhhhh! Mmmmmhhhh!" She continued to let out moans for about 5 minutes, durring which she ended up laying a few dozen, large eggs. You had several spiders ferrying eggs away from her and into a massive spider silk nest you had spun for her over the last 9 hours.
When she was finished laying the last of her eggs, Sarina was so tired that she no longer could stand. So you picked her up and set her inside the nest, then got into it with her. Her own tentacle clit was still full of energy, even if she wasn't, and fucked you like crazy. You probably are pregnant again, but this time you won't make Sarina a walking egg sack, and will just make a normal one. Or at least you will just stick it on to her back and not stuff it inside of her pussy like last time.