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.
Voldemort would likely be the main antagonist still. He might see Harry's potential as an animagus as a way to strengthen his own forces. His Death Eaters would also play a role in trying to convince Harry to join the dark side.
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.
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.
In many fanfics, Harry being a Dragon Animagus can bring about really cool storylines. For example, it could add a whole new layer of power to his character. He might be able to use the strength and abilities of a dragon in battles against Voldemort and his Death Eaters. It also could change his relationships with other characters. Some might be in awe of his new form, while others could be afraid.
Obviously, Harry Potter himself would be the central character. Then, Voldemort would be a major figure as he is the main representative of the Dark Side. Bellatrix Lestrange could also play an important role as she is one of Voldemort's most loyal followers. She might be the one to initially tempt Harry or guide him into the ways of the Dark Side.
One possible plot could be that Harry, influenced by his darker side, joins Voldemort to gain power. He might start to learn the Dark Arts from Voldemort directly. Another plot could be that Harry believes Voldemort has some secrets about his family that he can only learn by getting close to him. Also, it could be that Harry is tired of the Order's way and thinks Voldemort's methods are more efficient in achieving his own goals.
If Harry Potter joins the dark side in fanfiction, it would completely颠覆 the original story. His friends would become his enemies, and Hogwarts would be in chaos. The power dynamics in the wizarding world would shift, and Voldemort might see Harry as a valuable ally or a potential threat to his own leadership. It's a very interesting concept to explore in fanfiction.
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.