https://en.wikipedia.org/wiki/Codex_AleraThe inspiration for the series came from a bet Butcher was challenged to by a member of the Del Rey Online Writer's Workshop. The challenger bet that Butcher could not write a good story based on a lame idea, and he countered that he could do it using two lame ideas of the challenger's choosing. The "lame" ideas given were "Lost Roman Legion", and "Pokémon".[1]
www.fantasyliterature.com/author-interviews/jim-butcher/How did you come up with the original idea for CODEX ALERA? We’ve heard rumours that it involved a bet on whether you could combine the Roman empire and Pokémon… is that true?
The bet was actually centered around writing craft discussions being held on the then-new Del Rey Online Writers’ Workshop, I believe. The issue at hand was central story concepts. One side of the argument claimed that a good enough central premise would make a great book, even if you were a lousy writer. The other side contended that the central concept was far less important than the execution of the story, and that the most overused central concept in the world could have life breathed into by a skilled writer.
It raged back and forth in an ALL CAPITAL LETTERS FLAMEWAR between a bunch of unpublished writers, and finally some guy dared me to put my money where my mouth was, by letting him give me a cheesy central story concept, which I would then use in an original novel.
Me being an arrogant kid, I wrote him back saying, “Why don’t you give me TWO terrible ideas for a story, and I’ll use them BOTH.”
The core ideas he gave me were Lost Roman Legion and Pokémon… Thus was Alera formed.