The Dark Lady could be an ancient witch who was once a powerful figure but was banished for her dark practices. Maybe she was a rival of one of the great wizards in the past and now she's back to seek revenge.
It could be an original character created by the fanfiction writer. A witch who has been in the shadows for a long time, biding her time to rise to power.
The Dark Lady in Harry Potter fanfiction could be a variety of things. It could be a long - lost relative of a major character who has turned to the dark side. For example, she could be a distant cousin of the Malfoys who has her own agenda in the wizarding world that involves dark magic. She might also be an entirely new creation who fills a role that wasn't explored much in the original series, such as a female dark wizard who challenges Harry and his friends in ways different from Voldemort.
Well, in many 'Harry Potter and the Dark Lady' fanfictions, the plot often revolves around a new character, the Dark Lady. She could be an ancient evil that has been dormant for centuries and now resurfaces in the wizarding world. This might lead to Harry and his friends having to face new and unexpected challenges that are different from what they faced with Voldemort. Maybe she has her own dark magic and a group of followers who are causing chaos in Hogwarts and the wider wizarding community.
Harry's character could be consumed by anger and a thirst for revenge in a dark fanfiction. This would make him more aggressive and less empathetic. Luna, however, might use the situation to grow in strength. She could become a sort of moral compass for Harry, but at the same time, she might have to harden herself. Maybe she starts to study dark magic not to use it but to understand it better and find ways to counter it. This would change her from a carefree observer of the magical world to a more proactive defender.
One plot could be that in a dark version, Harry and Luna team up to expose a hidden, evil regime within the wizarding world that has been secretly controlling things from the shadows. They might have to use some rather unorthodox and darker magic to do so.
I'm not sure specifically as there could be many different interpretations of such a fanfiction. It might be about a mysterious or 'dark' female character within the Harry Potter universe. Maybe she has some sort of complex relationship with the main characters or a hidden agenda that unfolds in the story.
Often, the Dark Lady is depicted as powerful in magic. She may have a commanding presence and be skilled in spells that are either dark or rare. For example, she might be able to perform ancient and forbidden magic.
She could be an original character created by the fanfiction author. Maybe a witch who was secretly trained by a dark wizard but has a change of heart when she meets Harry.
In 'Harry Potter Dark Luna' fanfictions, often Luna's character is developed by giving her a troubled past. For example, she might have had some experiences with dark magic early on that shaped her in a different way. This could lead to her having a more cynical view of the wizarding world compared to her original, more whimsical self.