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 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.
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.
She might start by becoming disillusioned with the Jedi Order. Maybe she discovers some secrets that make her question their ways. Then, she could be lured by the power the Dark Side offers, like Darth Vader was. She'd likely start using more aggressive and unethical tactics in combat and in her relationships.
Cole is a complex character who has a history of being on both sides of good and evil. Phoebe, on the other hand, is usually a good - hearted character. So in this fanfic, the dynamic between these two main characters is crucial. Cole has the power and the intention to change Phoebe, while Phoebe is the target of Cole's actions.
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.
Some authors write 'Harry Turns Dark' fanfics because it allows for a different exploration of the character. Harry is usually seen as the hero, so turning him dark is a fresh take. It can also show how circumstances can change a person. For example, if he was constantly mistreated by the wizarding world.