Power is a common theme. As a great demon king, characters often have to deal with the acquisition and proper use of great power. For example, in 'Overlord', the main character has to figure out how to use his overwhelming power in the new world.
Another theme is leadership. Demon kings usually lead some kind of group or army. In 'Maou - sama, Retry!', the protagonist has to lead his demon minions effectively, which involves making decisions about resources, war strategies, etc. Also, the theme of redemption or the fight against one's own dark nature can be present. Some demon king characters may start off as purely evil but gradually develop a more complex moral compass.
Power. The protagonist as the great demon king is all about power and how to use it, gain more of it, and dominate with it.
Another theme is redemption or change. Sometimes, the demon king starts as an evil figure but through interactions with other characters, he may start to change his ways. For example, in some novels, the demon king may start to care about the well - being of his subjects more than just power. This can be seen in how he might make policies to improve the living conditions of the people in his domain. And also, the theme of mystery often exists. There are usually secrets about the demon king's past, or about the game world itself that gradually unfold as the story progresses.
A prevalent theme is redemption. Sometimes, the demon king may not be entirely evil, and there could be a chance for him to be redeemed, or the hero may have to struggle with his own dark side. Also, friendship and loyalty play a part. The hero often has comrades who support him, and the demon king may have his own followers. Their relationships within their respective groups are important aspects. In addition, the theme of sacrifice is often seen. The hero may have to sacrifice something precious to defeat the demon king or vice versa.
One common theme is the struggle between good and evil. The hero represents the good side, and the demon king the evil, and their conflict drives the plot. For example, in many novels, the hero tries to defeat the demon king to save the world.
The theme of change is also prevalent. His entire lifestyle, values, and relationships change. He might go from being feared to being respected or befriended. There is also often a theme of secrecy. He has to keep his past a secret in some cases, which adds an element of suspense to the story. He may struggle with when and how to reveal his true identity, especially when he starts to care about the people in his new life.
Power struggle is a common theme. Usually, the demon king girl has to deal with various forces that either oppose or try to use her power. For example, in many stories, there are other kingdoms or powerful entities that see the demon king girl as a threat.
Power. Usually, the reincarnated demon king has some form of great power from their past life that they can tap into. This power often makes them stand out in the new world they are in.
Power management is a common theme. The demon king often has to figure out how to use his power effectively in a more practical sense, not just for destruction.
Isolation can be another theme. Despite their power, demon kings may be alone or have very few they can truly trust. They might be feared and shunned by most of the world in the story, which can lead to their own internal struggles and character development.
Good vs. evil is also typical. The demon king is usually seen as the embodiment of evil, while the priest is associated with good, and the story revolves around their clashes or attempts at coexistence. For example, in some novels, the priest may try to redeem the demon king. This adds a layer of moral complexity to the story.