Sure, here's an example of a simple custom Pokemon game implemented as a single HTML file using JavaScript and enclosed in script tags:
When you open this HTML file in a browser or smartphone, it will start the battle between Pikachu (controlled by the player) and Charmander (champion opponent named "Reaper"). The battle log and outcome will be displayed in the browser's console. You can modify or enhance this code to include more features or interactions based on your specific game requirements.