Well, 'His Dark Lady' in a Harry Potter fanfic might be a character that the author has invented to add a new twist to the story. It could be related to the existing themes of good and evil in the Potterverse. For example, she could be a love interest for one of the main characters, but with a dark side that causes conflict. This could lead to all sorts of interesting plot developments, like moral dilemmas for the other characters or changes in the power dynamics within Hogwarts.
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.
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.
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.
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.
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.
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.
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.
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.