Sure! Here's an example of a simple horror-themed FPS game implemented as a single HTML file using JavaScript and enclosed in script tags. Please note that this is a very basic example and can be expanded upon to create a more immersive game experience.
In this game, you control a player represented by a red square. The goal is to avoid the green enemy square. Use the arrow keys to move the player on the screen. If the player collides with the enemy, the game will be over, and the "Game Over" text will be displayed.