* 魔王 (maō): This is probably the closest and most common translation. It literally means "demon king" and is frequently used in anime, manga, and games.
* 鬼王 (oniō): This translates to "ogre king" or "demon king." "Oni" (鬼) are typically depicted as more grotesque demons than "ma" (魔). This might be a better choice if you want a more brutal or frightening image.
* 魔神王 (majinnō): This translates to "demon god king," emphasizing a higher power and divinity associated with the demon.
* You could also use a more descriptive phrase, such as 悪魔の王 (akuma no ō) which literally means "king of devils." This is more literal and less common in fantasy settings.
The best choice will depend on the specific setting and the characteristics of your demon king. 魔王 (maō) is a safe and widely understood option in most cases.