Fic thought:
Jaskier gets tired of how Witchers are treated, he's spent decades with Geralt, watching him be spit on and payment shorted, despite his best effort his songs have done little to change public opinion, and other Witchers have it much worse.
It's after the Mountain, when he and Geralt part ways, not for long, it's never for long, no matter how mean Geralt can be in a moment, the Witcher always finds him again, a gift as a peace offering and apology both, that Jaskier stumbles upon an execution, a Witcher accused of leaving a monster unhunted, but Jaskier knows that no Witcher would do such, he knows that.
That moment changes everything, changes the very course of Destiny, because there is no way that Jaskier can stand idle and watch a Witcher be killed for no reason.
And so Jaskier saves the Witcher, a Letho of the Viper school, who in turns swears his life to Jaskier's protection despite, or perhaps because of, the Bard's protests.
Jaskier returns home for the season, trying to figure out what to do with the Witcher he now has in his service, and he hears of a Cat Witcher that's been enslaved by a King and it's then that Jaskier makes a choice and a plan.
It's easy enough, Letho has toppled kingdoms before, he's a Kingslayer after all, only it's not Letho's sword that takes the King's life, it's Jaskier's.
Jaskier takes over the Kingdom and Court quickly, roots out treason and lies thanks to the Cat he frees, who also pledges his service, he has no where else to go, his past removed before the King bought him, he remembers only what he is, a Witcher, and Letho who can hear lies.
Eventually the rest of the Cats and Vipers find their way to Jaskier's growing Kingdom, as do Elves and like creatures when they hear that Jaskier's Kingdom is a refuge for the odd and mistreated.
Jaskier's not entirely sure just how it happened but by the time Geralt comes to apologize, Jaskier has been a King and been running a Kingdom for at least a year and has an Army that is at least half Witcher, he's been getting marriage offers from people that turned his family down when he decided to go to Oxenfurt to learn music.
Yennifer offered to be his Court Sorcerer, for Melittle's sake.
Jaskier just wanted his Witcher and his people safe, he didn't intend for this to happen.
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Because accidental Warlord!Jaskier would be hillarious.
No one lets Geralt live down the fact that his Bard conqured a Kingdom so he'd be safe and treated better, also they definately call him Queen Geralt, or at least Lambert does.