As an AI text-based model, I can help you with providing a basic structure for a 3D open-world RPG game concept in the form of a single HTML file. However, please note that implementing a fully functional 3D open-world RPG game requires complex game engines and frameworks, which may not be feasible to achieve solely within a single HTML file with JavaScript. I can provide you with a basic outline that you can further expand upon and flesh out.
Here's a structure you can use as a starting point:
Please note that this is only a basic structure to get you started. Implementing a full-fledged 3D open-world RPG game with the described storyline would require additional libraries, frameworks, and more complex coding structures.
I hope this helps in getting you started with your game concept. Good luck with your development!