Some dark creatures could have magical powers that are not fully understood. They could create barriers or illusions to hide Harry from his pursuers. In the fanfiction, they may also use their knowledge of the dark arts in a positive way to counter the dark magic aimed at Harry. They might be able to sense danger before it arrives and take preemptive action to keep Harry safe.
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.
One consequence could be that the Death Eaters no longer sense his connection to Voldemort. This gives Harry an advantage in his fight against them as they can't track him through the Mark.
The main themes in 'dark harry dark ron fanfiction' are complex. One is the idea of moral ambiguity. Harry and Ron are made to face situations where the line between right and wrong is blurred. This challenges the traditional view of their characters. Another theme is the exploration of different magical paths. Instead of the usual light magic, they are exposed to dark magic, which changes their outlooks. And the theme of self - discovery in a dark context is also present. They learn more about themselves as they navigate through this dark world, which is different from their usual heroic self - discovery in the original story.
In some fanfictions, Harry might use powerful ancient magic that he discovers in a hidden library or from an old wizard. This magic could be specifically designed to counter the dark magic of the Dark Mark.
They could also physically and emotionally isolate him even more than they do in the original story. By cutting him off from any positive influence, like his friends at Hogwarts, they make him more vulnerable. Then, they could introduce him to some seemingly 'cool' or 'powerful' dark magic artifacts or spells, which might initially attract Harry's curiosity and eventually lead him down a darker path. For instance, they might find a way to get their hands on a dark magic book and leave it for Harry to discover.
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.
In canon, Harry is the hero who uses mainly light magic. But in 'Dark Mage Harry Potter' fanfictions, he uses dark magic. Canon Harry is more about following the rules and the good side. Dark Mage Harry breaks the traditional boundaries of magic. For example, he might use curses that are forbidden in the normal wizarding world.
It could be a fanfiction where Harry Potter is depicted in a darker or more morally ambiguous way. Maybe he's involved in actions that are not in line with the heroic image we usually have of him in the original series.
It could be any number of characters. It might be the Hogwarts faculty, like Dumbledore or Snape, who for some reason decide to keep Harry in the dark. Maybe they think it's for his own good or to protect some greater secret.