"Now cum, slave!"
You hear the words, feel the collar pulse once again, and feel it enter your thoughts, instantly breaking the barrier that held you from release.
Suddenly, the energy that built with each stroke of your master's cock, of each splash of his red hot cum on your face, of each pinch of your hardened nipples, is released. Sweet feminine sounds escape your mouth forcefully as wave after wave of orgasmic energy flows from your ergogenous zones to your neck, and finally into the collar. With each wave of energy it absorbs, it becomes warmer and tighter, causing you to sharply catch your breath between fading moans. At some point a combination of the pleasure and the light choking simply knocks you out, fuzzing your mind over with pleasure until nothing is left.
By the time you're able to think again you find yourself panting, kneeling on the floor in your ruined dress, covered in the cum of your ma-- The Usurper. You... you swallowed it! You made a man cum all over you. You just... pleasured the greatest enemy of your kingdom in your own throne room...
Indignified, you feel your heart rate begin to rise as you're filled with anger and shame. You shoot a defiant expression up towards the giant man standing over you just as the tight, hot feeling of the collar around your neck takes all your attention.
"That's all it took for you to feed the collar enough to break the first seal," The Usurper quipped, looking down at you, audibly holding back his amusement, "Now it's time for you to feel its true power!"
You break eye contact with him and look down, trying to catch a glimpse of the collar around your neck. All you can see is your defiled chest, still wet with the streams of cum that you... didn't... manage to swallow. Just as you remember your recent slutty behavior, you notice the pink glow of the collar on you. It pulsates slowly in a soothing rhythm as you feel it begin to push energy back into you with a warming, relaxing sensation. As wave after wave of increasing pleasure flow through you, your facial expression softens into a blissful smile, and your body begins to respond as well. The warmth spreads down through your neck, your chest, your belly, and finally out through your limbs, until your feel everything become one continuous sensation of softness and malleability.
Slowly panting, you start to feel new sensations in your body. You double over and wrap yourself in your own arms, unable to coordinate yourself. Your nipples tighten again, and swell, continuining until they're as big as you've ever felt them... then pushing out past that. Behind your growing nipples you feel an incredible pressure, and before you realize it your breasts start to swell up. They begin to push out of your silken dress, eliciting a happy coo from you as you joyously watch your body take a sluttier and sluttier form. You could release them from your dress, but instead you push them up with your crossed arms, encouraging them to keep on going, cooing again as they grow and stress your dress to the limit.
You feel pressure in your hips as the heat from the collar presses out insistently. Eager to encourage this part of your transformation, you begin to gyrate slowly, trying to release the pressure from your hips until--pop!--one side swells out, then--pop!--the other. Again and again you repeat the process, filling with pressure, releasing it, and basking in pleasure of your transformation. A part of you is filled with dread at the predicament, but the part of you in control is overjoyed that you're taking a sluttier form.
Warmth continues to mold you in other ways as well, first toning, then growing your thighs out to a size worthy of your new hips. Your calves tighten and swell into shapely forms, and clench your feet tightly, then release, as they change according to the collar's demands. Flowing through your arms, the transformative energy tightens and smooths them, making your already feminine arms even more delicate. Somehow your soiled princess gloves fit your arms even better now.
By the time the pink glow of the collar begins to dim and your body stops changing, you barely fit into your silky dress.