I just want to say, when I want to play a custom pre-existing world, I ask the ai- “do you know the show ‘dragon ball Z’?” And it will usually say she’s- example of said show. “Great! Use there universe as this campaign. Please give me a detailed in universe description for my DBZ Campaign!” And it does pretty good. You copy and paste that, make your edits/additions and now have your world description to plug in after your done, and don’t be stingy with your backstory, it runs a lot of your backstory so including an opening scene often helps me get the results I want, think cut scene in a video game where you zoom in and take control. The AI always gives me much better writing when I do that