It could be that the Dark Lord offers Harry something irresistible. Like a way to bring back his parents. Harry, being desperate to have his family back, might be tempted to join the Dark Lord in the fanfiction. After all, the love for his parents was always a strong driving force in his character, and this could be a major turning point that leads him to side with the Dark side.
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.
In such a fanfic, perhaps Harry Potter is magically manipulated. The Dark Lord could use some powerful dark magic to cloud Harry's judgment. Harry might then start to use his skills and knowledge, which were originally meant for good, to help the Dark Lord's evil plans. For example, he could use his Parseltongue ability to communicate with dark creatures for the Dark Lord's purposes.
In 'Harry Joins the Dark Fan Fiction', perhaps Harry is deceived by a powerful dark wizard who convinces him that the dark path is the only way to truly protect the wizarding world. This could lead to him abandoning his previous values. He could be trained in dark arts, and as a result, he might become an outcast among those he once called friends. However, there could also be a plot twist where he realizes his mistake and tries to redeem himself.
Well, perhaps Harry gets tired of being constantly in danger and feeling unappreciated in the wizarding world. He might be lured by the Dark Side with promises of power and safety. Voldemort could offer him a place by his side, showing him a different view of magic that seems more appealing in a warped way.
Well, perhaps in this fanfiction, Harry discovers his animagus form and the Dark Side somehow convinces him that his unique ability can be better utilized there. Maybe they promise him power and a new understanding of magic that the 'good side' has been suppressing.
One possible plot could be that Harry, feeling betrayed by the light side or lured by the power promised by the dark side, decides to join. The cane might be a symbol of his new identity or a magical tool given to him by the dark forces. He could start learning dark magic and be used to carry out their evil plans.
Well, one possibility is that she gets frustrated with the slow progress of the 'good side'. Maybe she sees all the bureaucracy and inefficiency in the fight against the dark forces and decides that the dark side can offer a more direct way to achieve her goals, like a more efficient magical society. But this would be a huge shift from her original character.
In the fanfiction, Luke could be influenced by his family ties to the dark side. If he discovers some hidden connection to Vader or Palpatine that he didn't know before, it might make him question his place on the light side. He could think that joining the dark side is a way to understand his true heritage better and gain more power to protect those he loves in a different way than the Jedi would allow.
In this fanfiction, it could be that Ezra is deceived by a powerful dark side figure. He might start to believe that the Jedi way has failed him and that the dark side can provide a more effective means to achieve his goals. This could lead him to make choices that gradually draw him deeper into the dark side, like using dark side powers and distancing himself from his former Jedi allies.
Well, it could be that her joining Voldemort causes a huge shift in the balance of power. The wizarding community would be in chaos as they try to figure out how to deal with this new threat. Dark Fem Harry might also start to influence Voldemort in unexpected ways. Maybe she convinces him to use more psychological warfare rather than just brute force. And she could be the one to find and destroy some of the remaining Horcruxes of other wizards, believing that it would make Voldemort even more powerful and thus her position more secure.