Sure, here's a sample implementation of a simple 3rd person shooter game with a horror demon theme, using JavaScript and HTML in a single file:
You can save the above code as "horror_demon.html" and open it in a browser to play the game. The player (blue square) moves based on mouse movement and shoots bullets (red rectangles) by clicking the mouse. The goal is to avoid the demon (red square) and defeat it by shooting multiple bullets at it. If the player collides with the demon, the game is over.