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.
Hermione and Ron would definitely be main characters. They would be the ones most affected by Harry's decision to join the dark side. They would likely try to figure out why he made such a choice and try to stop him. Dumbledore, even if he's dead in the original story, could still have an influence through memories or prophecies. Voldemort would also be a central figure as he is the main representative of the dark side that Harry is joining.
It could be Bellatrix's younger sister who has a different view on the dark side compared to Bellatrix. She might be more conflicted and her relationship with Harry could start as an enigma that draws Harry closer to the dark side.
Well, one possibility could be that Harry is drawn to the allure of the dark side because of a love interest there. Maybe a mysterious dark witch or wizard catches his eye and through that relationship, he starts to see the dark side in a different light. For example, they might introduce him to a more 'liberating' way of using magic according to the dark side's view, which slowly makes him question his previous beliefs.
The teachers at Hogwarts, like Dumbledore (if he were still alive in the fanfic), would be deeply concerned. Dumbledore always saw great potential in Harry for good. He would probably try to understand what led Harry to this path and would use all his wisdom and magic to try and turn Harry back. The other teachers would also be on high alert, fearing for the safety of the students and the future of the wizarding world.
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, 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.
Perhaps he was deceived. There might be a powerful dark wizard who managed to convince Harry that the dark side had a more 'just' cause. This wizard could have used false information about the history of the wizarding world and how the dark side was initially misjudged.