Who are 'they' in the 'they kept harry in the dark fanfiction'?

Well, it could be about a situation where some characters keep Harry in the dark, perhaps about his true identity, his powers, or some secrets within the fanfiction's fictional world. Without reading it specifically, it's hard to say exactly.

Maybe an amnesiac character. They've lost their memories and don't remember anything about the vampires and other supernatural beings. They are the perfect candidate to be kept in the dark as they have no prior knowledge to start with. Their journey to regain their memories and discover the truth about the Vampire Diaries world could be the main plot.

Perhaps vampires. Vampires are often powerful and if they have a reason to protect Harry, they could be very effective. Maybe they have a past connection with Harry's family or they see a future benefit in keeping him alive. There could also be creatures like basilisks. If they are tamed or controlled in the fanfiction, their deadly stare could be used against Harry's enemies instead of against him.

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, it could be about a character who is kept in the dark regarding the true nature of the vampires in Mystic Falls. Maybe they are new to the town and everyone is hiding the fact that vampires exist from them. And then they start to notice strange things like people having super strength or not aging, and the story could be about their journey to discover the truth.

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.

Dumbledore could be seen as the main antagonist from Harry's new perspective on the dark side. Dumbledore represents all that is good in the wizarding world, and if Harry has turned to the dark, Dumbledore would be the one trying to stop him, to bring him back to the light. So in this context, Dumbledore becomes the opposition that Harry has to face.

Obviously, Harry himself is the central character. Hermione and Ron are also often present. In some stories, they try to save Harry from the darkness, while in others, they become his enemies as he turns dark. Dumbledore may also play a significant role, either trying to guide Harry back to the light or being unable to stop his descent into darkness.

The characters in the 'dark trio' in the fanfiction are likely to be those who have a propensity for the dark arts or have some sort of malicious intent. It could be that the author takes minor characters from the original series and gives them more prominent roles within this dark context. For example, Crabbe and Goyle could be developed further into more menacing characters, or perhaps some of the Death Eaters that were not as well - explored in the original books could be part of this trio.