Geralt/Jaskier/Eskel smut, which I will never write, so have some modern fantasy au filler plot instead...
Jaskier, struggling musician extraordinaire, has made a lot of friends in high, low, and strange places over his years traveling the continent trying to make a name for himself. So when one of those friends, an undercover operative with the Redanian Special Forces, offers Jaskier generous compensation in return for infiltrating an underground fight club/human trafficking operation, Jaskier agrees even if it means playing his least favourite part: Julian Alfred Pankratz, son of crooked and filthy rich politician, Alfred Pankratz de Lettenhoven.
Jaskier finds the place with ease and makes an entrance worthy of his father's son and starts making the rounds so his friend can get a good look at the fight club floor and all its guests via the button cam sewn into his rather lurid silk shirt.
He throws around enough of his borrowed cash to earn a peak backstage at the merchandise - dozens of terrified young men and women, many with slightly odd features to indicate shared blood with some fantastical creature.
His character's slimy grin cements itself on his face lest he throw up in disgust as he forces himself to make a show of inspecting the goods, poking and teasing and flirting with the poor caged things so the organiser will think he's just as vile and as interested as his other customers.
And then he's shown the main attraction: two bruised and bloodied men with golden eyes and scars from claws, teeth, and blades littered over their bare torsos. The fight club organiser is going to throw these two mighty warriors, who Jaskier hadn’t realised existed outside of history texts, into the makeshift ring to fight a few wannabe monster slayers who’ve all paid for the privileged of going a few rounds with a real live witcher.
“You wanna have a go?” the organiser teases.
“Absolutely,” Jaskier cheerfully leers. “Oh, you mean fight?” he guffaws. “No, my good sir, I was thinking something a little more intimate but just as athletic.”
The organiser snorts with good humored disgust. “That’s a good way to get your dick ripped off.”
“But surely you have a way to make them behave,” he whines petulantly. “I can’t imagine they’re sitting quietly in these cages because the rent’s cheap. Even if all the stories about witchers are half true, they’d still be strong enough to break free of something as mundane as iron bars.”
“The shock collars keep em’ docile,” the man brags, pulling a small remote from his pocket and waving it about. “Cages and the ring are all wired up too. They take one step outside ‘em and they get lit up with enough electricity to power a small city.”
Jaskier resists the urge to punch the organiser in his greasy face and instead assures him that he’s definitely interested in the auction later in the evening and then returns to the fight club floor to watch two roided up idiots beat the snot out of each other and impatiently wait for his friend and his heavily armed colleagues to raid the building.
To while away the minutes he inspects - out of range of his button cam - the remote he stole from the organiser’s pocket. When the police raid the building Jaskier deactivates the witcher’s collars and hopes they managed to escape and vanish back into myth and legends where they belong.
When Jaskier arrives home, half a day later and a thousand dollars richer, he’s more than a little surprised to see the two witchers loitering in his living room wanting to thank him for his assistance and ask him with the hell he wants in return for it.