As an AI language model, I can provide you with an example of a simple game implemented as a single HTML file using JavaScript. However, due to the complexity of creating a full-fledged sequel to Undertale, it would go beyond the scope of a single response. Nonetheless, I can help you create a basic structure and guide you through how to implement game mechanics. Here's an example of a game menu using pictograms for a smartphone operation:
In the provided example, we create a simple game menu with a title, an Undertale 2 logo represented by an image, and a "Start Game" button. When the button is clicked, it triggers the `startGame()` function, which currently displays an alert message as a placeholder for starting the actual game.
Please note that this is just a starting point for creating a game and that a complete game like Undertale would require a significant amount of additional code and assets. However, this example should give you an idea of how to structure a simple game menu using HTML, CSS, and JavaScript.