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.
One possible plotline could be that Harry is influenced by a dark artifact. For example, he discovers a cursed object that slowly corrupts his mind, making him turn against his friends. He might start to use dark magic to achieve his goals, believing that the end justifies the means.
One way it could be plausible is through the use of dark magic manipulation. Voldemort is a master of dark magic. He could perhaps use some form of mind - altering spell on Harry. If he managed to get Harry in a vulnerable state, like when he was feeling isolated or betrayed, the dark magic could seep in and start to change his nature. For example, if Voldemort made Harry believe that his friends at Hogwarts had turned against him, Harry's sense of justice and goodness might be warped, leading him to the dark side.
In a 'Harry Potter turns to the Dark Side' fanfiction, the plot might involve Harry being influenced by a powerful dark object. For example, a cursed artifact that makes him see the Dark Arts in a different light. He could start to believe that the ends justify the means and that the Dark Side has been misrepresented all along. As he delves deeper into the dark side, he begins to turn against his old friends, thinking they are too naive and weak to truly change the wizarding world.
The exploration of the dark side of magic is also a theme. We get to see how the dark side can be so alluring. In these fanfictions, Harry may start to explore spells and powers that he would never have considered before. And the theme of betrayal is often there. Harry might feel betrayed by the wizarding world that he once fought for, which makes it easier for Voldemort to turn him.
One idea could be that Harry discovers a dark secret about his family that leads him down a dark path. For example, he might find out that his ancestors were involved in some really evil magic and he starts to explore that side of magic. Another could be that the influence of Voldemort somehow remains and corrupts Harry's mind over time. Maybe the Horcrux inside him wasn't completely removed and it starts to change his way of thinking.
Voldemort could still be a major antagonist, but in a different way. Since Harry is turning to the dark side, Voldemort might see him as a threat to his own power and try to eliminate him. Another could be Dumbledore. If Harry turns dark, Dumbledore would surely oppose him, trying to bring him back to the light. So Dumbledore could be an antagonist from Harry's new dark perspective.
Well, in this type of fanfiction, perhaps Harry could be lured to the dark side by a new, extremely charismatic dark wizard or witch. This new character might offer Harry power, knowledge, and a sense of belonging that he feels he lacks in the wizarding world. They could manipulate Harry by appealing to his insecurities, such as his status as an orphan and his feelings of being different from other wizards. And then there's the idea that the dark side might seem like a more efficient way to achieve his goals. If Harry is frustrated with the slow progress of fighting Voldemort through traditional means, he could be tempted to use dark magic to end the threat once and for all.
One popular 'Harry Turns Dark' fanfic could be 'The Darkened Heir'. In this type of fanfic, often the author explores how Harry's experiences could have pushed him towards the dark side. Maybe the betrayal he faced from some in the wizarding world made him bitter. Another could be 'Dark Harry's Ascent' where the story might show how Harry uses dark magic to get revenge on his enemies. And there's also 'Harry: The Dark Lord Reborn' which could play with the idea of Harry taking on a more sinister role similar to Voldemort but with his own motives.
It could be Dumbledore. If Harry turns to the Dark Side, Dumbledore, who has always been a guiding figure for Harry, would be the main one to oppose him. Dumbledore represents the light and everything Harry is now against. Dumbledore would try to stop Harry from causing chaos and destruction in the wizarding world.