One key theme is the struggle between good and evil within Arthur. Since it's a 'dark arthur' concept, his darker nature often battles with his better self, and Merlin is caught in the middle. Another theme is loyalty. Merlin's loyalty to Arthur is tested as Arthur's actions become more unpredictable in these darker interpretations.
Power is a significant theme. In 'dark arthur x merlin fanfiction', the power dynamics between them change. Arthur may misuse his power, and Merlin has to figure out how to handle that. Also, the theme of secrecy comes into play. Merlin has to keep his magic a secret in a more dangerous situation with a darker Arthur, which adds a lot of suspense to the stories.
You could check out 'Shadows of Arthur and Merlin'. In this fanfiction, the author creates a rather gloomy and intense atmosphere. It shows how Merlin tries to deal with a more menacing Arthur, and their interactions are full of tension and unexpected twists. It's really a fascinating read if you like this kind of pairing.
The theme of betrayal is often seen. Maybe Merlin is wrongly accused of betraying Arthur due to his use of dark magic for a good cause. Or Arthur might be seen as betraying his own ideals under the influence of some dark power. It adds a lot of drama and tension to the fanfiction. Also, the exploration of a post - apocalyptic Camelot where Arthur and Merlin have to fight against dark forces to rebuild their world is a cool theme. It gives a whole new dimension to their characters and relationship.
One common theme is the exploration of a new kind of relationship. With Merlin being pregnant, it changes the power dynamic and the way they interact. Another theme is acceptance, both from themselves and from the people around them.
Love and obsession is also a major theme. The line between Arthur's love for Merlin and his obsessive yandere behavior is often blurred. The fanfictions usually explore how Merlin responds to this love - turned - obsession. Sometimes Merlin is confused, other times he tries to understand and help Arthur deal with his extreme emotions.
Well, a lot of merlin x arthur fanfiction focuses on the power dynamic between them. Since Arthur is a king and Merlin his servant, exploring how that changes when their relationship becomes more than just master - servant is a big theme. And of course, there are themes around protecting each other. Merlin always protects Arthur with his magic secretly, and in fanfics, this can be elaborated on more, like in situations where Arthur is aware of Merlin's efforts.
Love, in different forms, is also a theme. It could be platonic love with a possessive edge, or it could be budding romantic love. The fanfiction may explore how this love affects their actions and decisions. For example, Arthur's possessive actions might be his way of expressing love without fully realizing it. And Merlin's reaction to it can be a way to show the growth of their relationship.
Another theme is the forbidden love aspect. Given their different roles in society, their relationship might be seen as unacceptable in the context of the story's world. This adds a layer of drama and tension. Some stories also focus on how they overcome these societal barriers. Moreover, magic is usually a big part of it, as Merlin is a powerful magic user. His magic can either be a source of conflict or a way to bring them closer together.
One common theme is friendship. Their relationship is often at the heart of the stories, showing how they support and grow with each other. Another is loyalty. Arthur and Merlin are extremely loyal to each other in the original series, and this is often emphasized in fanfiction.
One common theme is the exploration of gender roles. Since Merlin is female in these fanfics, it often challenges the traditional gender roles in the Arthurian legend. For example, how a female Merlin would be perceived as a sorcerer in a male - dominated society like Camelot.
One common theme is the struggle with unspoken love. Arthur's jealousy often arises from his love for Merlin that he can't or won't admit. It's a way of showing his possessiveness in a romantic sense without being fully aware of it himself.