When writing 'dark yoh fanfiction', it's important to develop a strong plot. Start with an interesting premise like Yoh being trapped in a never - ending nightmare or being hunted by a powerful and malevolent force. Then, build on this premise by adding layers of conflict and character development. Yoh's journey through this dark scenario should be filled with challenges that test his limits. You can also introduce new characters that either help or hinder him in this dark world. And don't forget to use vivid descriptions to set the mood, whether it's a gloomy forest or a desolate cityscape that serves as the backdrop for the story.