As you go to your bedroom to recuperate your old legs, you hear a rattling sound. Looking in its direction, you're surprised to see a rattlesnake barring your path. While you searching something nearby you could defend yourself with, your magical sword inexplicably appear in your hand and you simply decide to try impaling the beast. With a speed that surprise you, you cut the sake in two. On one side you have its body, and on the other the head.
You delicately push the unmoving head with your sword before looking at the body, and finds with dread that it is a Mojave snake, the most venomous rattlesnake of the world. At first you were simply going to make sure the infernal crawler wouldn't recompose itself, but seeing how it is still alive, you decide to conduct a little experiment.
You start by cutting its head in two with your sword, separating its left side from the right so you can better see what you're doing and diminish even more the risk to be bitten. Then you delicately separate each fang, venomous gland include, from the snake and delicately put them in a tissue and make sure you didn't damage the glands before cutting off your own fangs with the magical blade and put them in the serpents mouth.
As soon as your human fangs are in the two half-heads of the snake, they adapt by shrinking.
Satisfied with your little test so far, you decide to put the snake's own fangs in your mouth and go to the nearest mirror, which put you in the bathroom. As you open wide your mouth, you feel new muscles deploying your new venomous fangs. Because a human mouth can't open as wide as a snake's one, your new fangs adapted by becoming shorter, but they also became as large as your old ones and retained their hook like aspect.
Looking in the medical cabinet you see it now have a consequent anti-venom stock. Looking at it more precisely you see it's Mojave anti-venom. It make sense since you produce the same venom, you just hope you're not sensible to said venom and this stock is just in case you accidentally put a fang in your girlfriend or something.
The now very impatient and annoyed brunette yell "What take you so long?!?"
"Sorry, your legs had tried to escape, and once I found them again they wouldn't let me grab them" you lie.
As you exit, you see the still moving snake body on one side, and the two half-head on the other. First you reassemble the head and put it in your pocket, then grab the body and go to your room. You were about to deposit the two serpents pieces somewhere and give the woman downstairs your old legs, but realize she might contribute to another experiment.
"Okay, that's it! I'm coming and I swear, if I find you using my dick as your sex-toy I'll make you look like a cubist sculpture!" Only having the top part of one's body make going upstairs a rather awkward and distracting action, which you decide to use. Sneaking up on the distracted woman, and connect the snake-body to her stump.
Instantly, you see the long reptilian tail grow and grow and grow. To your surprise the feel of the brunette's scales change, and not only because she stay warm-blooded. Somehow, the top of her tail in the prolongation of her back shape itself to imitate a woman's shapely buttock, while on the front side appear a barely guessable but still present slit where the entrance to her vagina would have been if she still had her woman legs.