The 'onmyoji novel' might be centered around the concept of onmyoji. In Japanese culture, onmyoji were practitioners of a traditional esoteric cosmology, involving the study of astronomy, divination, and the art of controlling spiritual beings. So the novel could be about their training, their battles against evil spirits, and their relationships within the society of that time.
Well, if it's an 'onmyoji novel', it may tell stories of the characters who possess the power of onmyoji. It could include their quests to protect the world from dark forces. There might be different clans or factions of onmyoji, each with their own unique abilities and ideologies. The novel might explore how they interact, compete, or cooperate with each other to face the common threats in their fictional universe.
I'm not entirely sure specifically as 'onmyoji' can refer to various things. It could potentially be a novel related to the Onmyoji game, which often features Japanese folklore elements like yokai, onmyoji (yin - yang masters) and their adventures in a fantastical world.