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.
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 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.
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.
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.
In some fanfictions, the 'His Dark Lady' might be a character who was once under the control of Voldemort but has broken free and is now seeking redemption with Harry's help. This could involve adventures where they have to face remnants of the Death Eaters together, and in the process, their relationship deepens in unexpected ways. For example, she could use her knowledge of the dark arts, which she acquired while being on the wrong side, to help Harry in his fight against the remaining threats in the wizarding world.
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.
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.