Mark was soaking in the smokey, late night atmosphere of the bar. The plan for the night? The sweet calmness and indifference of bitter alcohol. Total relaxation.
The plans were, of course, ruined soon enough by a stranger who approached him and sat down next to him. A woman, looking around his age, her very noticeable breasts and hips spectacularly filling her black dress. She eyed him top to bottom before finally speaking:
"What's your name, pretty boy?"
It took him a solid moment to realize he is being spoken to.
"Oh me? I'm Mark."
She chuckled in a classy way.
"You are hilarious, I like you. Fancy a drink?"
Mark, still caught off-guard, only nodded. In response, she raised her hand slightly and looked at the barman.
"Two of the usual please."
"The usual? Sounds like you're a regular here. Also I don't know your name yet."
"You will call me Carmilla" she smiled. "And yes, I like coming here from time to time, it can be very fun."
"Carmilla... That is an exotic name." Mark's thoughts have at this point become completely absorbed by this strange encounter.
"Some might say I'm a fairly exotic woman. But oh well, you'll find out soon enough for yourself."
"By myself? What do you mean?"
"Are you really that clueless?" She laughs. "I'm going to have sex with you. Unless you have better plans."
Surprised and flustered, Mark takes a moment to regain composure. "Yeah, I'm down for that. You look great in this dress."
"Oh thank you." She smiles slightly. "And you look surprisingly cute for a grown man getting flustered by the mention of sex. Are you a virgin by any chance?"
Mark gives her a shameful look. Their drinks are served.
"Aww, poor thing. Well then, lucky you. Bottoms up and let's go upstairs, have some fun." She raised her glass in a pseudo-toast, then drinks it up. Mark also empties his glass. She stands up and nods at the barman.
"Follow me."
He did follow her upstairs, impressed by her swaying ass. They arrived in a dimly lit room, with some general furniture one could expect from a hotel room, and a big bed as a center piece. She grabbed his face from below and guided it for a kiss.
"Time to start the fun, pretty boy."
She then gently pushed him to fall onto the bed.
"Let's see what I'm dealing with today. Take off your pants."
Mark did so without question. She playfully clawed at his stomach with her long nails, before yanking his boxers down. She smiled.
"Nice. The cock is pretty average, but those are some juicy balls. I want them in my mouth right now." She knelt before him and started to kiss and eventually suck on his testicles while caressing his chest and stomach with her hands. The fullness of her lips and the additional action of her tongue really worked together for surprisingly effective pleasure. Eventually she let go of the balls filling her mouth and asked:
"Which testicle is your favourite?"
"What?"
"If I were to bite one of them off, which would you miss more."
Mark takes a moment to think about it.
"The right one."
"Good boy" she chuckles. "Truth is, I roofied your drink with a drug that makes you go along with whatever I want to do to you. Until sober again, you have no choice but to obey me. Now, let me have some fun." 
She started to bite on and tug his right testicle, increasing in force over time. "Oh shit, she will bite it off" he thought in panic. But before any serious damage could be done, she stopped and started laughing.
"That fear and panic in your face. Did you really think I was going to do it?" She laughed louder than before.
"Let me make this up to you" she continued, then got up and started undressing, slowly and sensually. With her tight dress gone, her ass and breasts turned out to be even bigger than they seemed at first. She removed her bra while looking lustfully over the shoulder, then made sure to slowly bend over as she removed her panties. A beautiful woman, fully confident in her body.
She knelt again, this time taking his cock in her mouth, as his balls were comfortably encased in her breasts. Her expert head movements and tongue play made him cum almost instantly. She climbed atop of him, bent down to kiss him and fed him his own semen in a passionate kiss.
"You're in my world now, pretty boy. Enjoy your stay. And call me godess."
"Yes my godess..."
"Good, good" she caressed his cheek. "Now be a good boy and return the favor" she said, then sat on his face. "Please your godess. If you fail, I will castrate you. Failures don't get to spread their genes." She inserted his balls into a custom device that looked like an oversized cigar cutter. "One little click and your balls are gone. Better start licking."
As commanded, Mark put his whole effort into pleasing her pussy with his mouth. Calm at first, she eventually started gasping, then moaning - Mark was doing his job well. As she neared her climax, her hips began to move more on his face, in a humping motion. Finally, with a loud moan, she has reached her climax and... accidentally activated the mechanism.
A soft click, then a metalic clink, then Mark's muffled screams. Carmilla still kept his mouth shut by sitting on it. She began to pet his head.
"Shhhh... You did great. You just lost your manhood because you made me cum too hard. That is the highest honor I could give a man. I will even keep your balls to remember this day."
She slowly uncovered his mouth. "Thank you my godess..." he said, his mind completely subservient to her.
"Good answer" she said. "I guess now that you are removed from the gene pool, I can fuck you properly. Didn't want any of your pathetic seed in me. But you make for a good pleasure eunuch. Enjoy."
She impales herself onto his dick and starts riding it, picking up speed as she goes. In the meantime, her hands wander from his chest up to his neck, also squeezing her impressive tits between her elbows. As she choked him, the pleasure became amplified. Shaking out of pleasure, he came.
"There. Took your balls AND your virginity. Now get the fuck out of here before I cut that cock off too!"

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Story written for this sub. 
Glad to see there is a replacement for r/castration_erotica, pity that many stories I published there are now completely lost. Hope you enjoy!