Your body is wracked with all sorts of wicked pain. Your head is pounding, and your skin feels like it's on fire. You watch through your watering eyes, barely making out that yoru skin is drying up like fall leaves and falling away from you in large flakes, leaving glistening scales behind, dark on your back surfaces, very pale on your front/inner surfaces.
Your skull cracks violently and begisn to reshape, your face pushing out into a destinct snake snout, complete with very shrp, very venomous fangs. Your eyes stop tearing as they lose the ability to do so, becoming reptilian slitted pupils. Unable to close your eyes, you must watch as your changes continue. Your senses are altering. You can still see fairly well, a major benefit of becoming a king cobra as opposed to other snakes, but you find that you are getting much more information from your flickering forked tongue.
You are distracted from this, however, by feelings of excrutiating pain from your torso. It's altering and reshaping in disturbing ways. You swear you can feel your internal organs changing, becoming longer, stretching downward. Your torso is narrowing, becoming very serpentine in it's own right, your arms, which remain, looking like odd attachments to the sides of your torso.
These changes continue down your abdomen, altering everything in their path. Your hips and rear change dramatically, your body now flowing smoothly from torso to tail, your ass as you knew it gone, only a slit on your underbelly there to perform the task. Your maleness has been drwn into a moist, soft genital pouch, and feels like it may be altering as well.
Your hands now have only three fingers and a thumb, ending in sharp, curled talons. Your feet are changing similarly, becoming long, reptilian, three-toed feet attached digitigrade-style to legs that, like your arms, look more like they are attached to your lengthening serpentine body rather than being a flowing part of it. Your body extends further, for several more feet, until you are easily twelve feet from nose to tailtip, still your same height, but much much more flexiible and strong, with venom that could put even very large creatures down in a matter of moments.
As the pain finally recedes, you have to come to terms with what has happened. You are now an anthropomorphic king cobra, a cannibalistic, large, powerful snake. Did everyone undergo changes like this? You do not know. The world outside could be a much different place now... On the other hand, it could just be you. What do you do?