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.
Well, one possibility could be that Harry, feeling betrayed by the Order of Phoenix or perhaps influenced by some dark magic artifact, starts to question the motives of the good side. He might see how the Dark side offers power and a different kind of 'freedom'. For example, Voldemort could manipulate Harry by showing him how the wizarding world could be united under his rule, eliminating the in - fighting among pure - bloods, half - bloods and Muggle - borns, though of course this would be a false promise.
It could be Dumbledore. In a twisted take on the story, if Harry joins the dark side, Dumbledore would be the one trying to stop him. Dumbledore has always been the guiding force for the good side, and Harry's defection would be a huge blow. Dumbledore would then have to face the difficult decision of either trying to save Harry or stopping him by any means necessary, making him the antagonist from Harry's new 'dark' perspective.
One great fanfiction that explores the dark side in the Harry Potter universe is 'The Darkening of Hogwarts'. It delves into what would happen if dark magic seeped deeper into the school. The characters are faced with moral dilemmas and the story shows how the allure of the dark side can be both tempting and terrifying.
Perhaps Harry discovers some dark secrets about the wizarding world that make him lose faith in the good side. For example, he might find out that some of the so - called good wizards have been involved in immoral acts. This disillusionment leads him to think that the dark side might be a better option. As he joins them, he starts to abuse his power, using dark magic to get revenge on those who he believes have wronged him.
Well, one possibility is that Harry is tricked into joining the dark side. Maybe Voldemort uses some dark magic to cloud Harry's mind and make him believe that the dark ways are the right ones. He could start using dark spells and turn against his friends.
Hermione would be devastated. She has always been Harry's loyal friend and believed in the fight against the Dark Lord. She would probably try to convince Harry to come back to the right side, reasoning with him about the evil nature of the Dark Lord's actions. Ron, on the other hand, would be both angry and confused. He might initially think it's some sort of joke or that Harry has been tricked.
If the dark side saves Harry in fanfiction, it could open up a whole new world of story possibilities. Maybe it would involve Harry having to confront his preconceived notions about good and evil and find a way to balance the powers within him.
In some fanfiction, Harry Potter can be portrayed as 'dark' and a 'bearer' in very different interpretations from the original series. Fanfic authors often like to explore alternative character arcs and personalities.
Some fanfictions in the dark side of 'Harry Potter' might focus on the characters' inner demons. For example, a story about Harry being corrupted by the power of the Elder Wand. It could depict his gradual descent into a darker version of himself, struggling with the allure of power and how it changes his relationships with his friends.