Sure, here's an example of a simple game based on the concept of "The Backrooms" using JavaScript in a single HTML file:
Welcome to The Backrooms!
In this game, you explore a series of interconnected rooms, making choices on which doors to go through. The descriptions and options change based on the current room you are in. The game ends when you either find the exit or get lost forever.
Please note that this is a simple and basic implementation, and you can modify and expand it in any way you like to suit your specific requirements or desired features.