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.
Maybe it's a long - lost relative of one of the main characters. For instance, a distant cousin of Snape who has turned dark and now wants to take revenge on the wizarding world for some past wrong. This would connect her to the existing lore in an interesting way.
She could also be a character from another magical realm that has entered the Harry Potter universe. A powerful sorceress from a different dimension who has a different set of magical rules and ethics, and her presence disrupts the balance in the wizarding world of Harry Potter.
The Dark Lady could be Bellatrix Lestrange. She is a powerful and fanatically loyal Death Eater. If Harry Potter were to become a fan of her in a fanfic, it might be because he is somehow tricked into seeing her as a symbol of power and freedom from the traditional wizarding rules.
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.
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.
Since I haven't actually read this particular fanfiction, it's hard to say for sure. But based on the title, I would assume the 'dark lady' is a main character, and perhaps there's a male character whose perspective the story is from. There could also be appearances from the classic Harry Potter characters like Harry, Hermione, and Ron as they interact with this new 'dark lady' character.
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.
One possible plot for this fanfic is that the Dark Lady has a secret past connection with Harry Potter's family. When Harry discovers this connection, he becomes curious about her. As he delves deeper, he is exposed to her alluring but dangerous world. He might be tempted to embrace her ways as he sees some perceived 'truths' in her actions and beliefs. This would lead to a major internal conflict within Harry as he struggles between his old values and the new, darker allure.
In 'Dark Lady Fem Harry Potter Fanfiction', the central figure is Fem Harry as the Dark Lady. Hermione is frequently present. She could be trying to bring Fem Harry back to the light side or be in a power struggle with her. Ron is there too, sometimes as a loyal friend to Fem Harry, other times as someone who is wary of her new dark persona. Additionally, characters like Draco Malfoy often play a part, perhaps as a rival or an unexpected ally to the Dark Lady Fem Harry. The Hogwarts professors are also important, as they try to deal with the situation of having a Dark Lady in their midst.
The plot might be about their discovery of a long - lost dark magic artifact. Harry and Daphne hear rumors about it and set out on a quest. Along the way, they face many challenges, like other dark wizards who also want the artifact and the Ministry of Magic's attempts to stop them. As they search, their relationship deepens. Once they find the artifact, they use it to gain more power, but also realize it has a dark side that could destroy them if they're not careful.
In some 'Harry Potter Dark Lady fanfiction', the Dark Lady might be a character who has a complex backstory. Maybe she was once a good witch who was wronged and turned to the dark side. This would add depth to the character and make the story more engaging as we see her transformation.