In dark arthur fanfiction, Arthur is the main focus, of course. But characters like Guinevere can also be important. In some stories, she has to deal with the changed Arthur. Lancelot might also be involved, perhaps as a rival or an ally in the complex world of a dark Arthur. Mordred is another character that often shows up, as his relationship with Arthur is already complex in the original legend and can be further explored in the dark versions.
One great 'dark arthur fanfiction' is 'The Fall of Camelot'. It explores Arthur's descent into a darker version of himself due to a powerful curse. Another is 'Darkness within the King' which delves into Arthur's inner turmoil and how it affects his rule. And 'Arthur's Shadow' shows a more menacing side of Arthur as he battles against his own dark nature.
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 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.
I'm not sure about the absolute best - known ones specifically for Merlin Dark Arthur fanfiction. However, some authors who are popular in the broader Merlin fanfiction community might also write in this darker sub - genre. Some might be those who have a large following on fanfiction platforms like Archive of Our Own. They often create stories that delve into the darker aspects of Arthur and Merlin's relationship and the world they inhabit.
Another theme is the exploration of their different personalities. Arthur is usually more responsible and DW can be more carefree. Fanfictions play with this by putting them in situations where their personalities clash at first but then they find a way to complement each other. For instance, in a story where they are planning a party for their parents, their different approaches initially cause problems but in the end lead to a great celebration.
The 'Redemption' trope is also seen in some fanfictions. Maybe Arthur has made some mistakes in the past, and Merlin helps him to redeem himself. Or vice versa. It's a way to explore the characters' growth and development. And then there's the 'Battle Against Evil' trope where they fight side by side against dark forces, which really showcases their bond and loyalty.
I really like 'The Sword in the Stone' as a fanfiction - like take on Merlin and Arthur. It shows Merlin as this wise and mysterious figure guiding Arthur on his journey to become king. There are also some great interactions between the two characters that show the deep bond they share. You can find it in many libraries or online bookstores.