Common themes include good vs. evil, of course. But it's not always so black and white. There can be moral ambiguity. Another theme is the idea of fate. Are the hero and the demon king fated to fight? And how do they deal with that? Also, the concept of power corruption is often explored. The demon king may be corrupted by power, or the hero may face the temptation of power as he gains more strength. And the theme of perseverance is essential, as both sides usually have to keep going in the face of great difficulties.
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.
One common theme is the battle between good and evil. The hero represents good and the demon king represents evil, and they fight for the fate of the world.
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.
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.
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.
One common theme is the struggle between good and evil. Since the hero is a demon, there's often a conflict within them about whether to follow their demonic instincts or do good. Another theme is redemption. Many of these fanfics show the demon hero trying to make up for past wrongs.