One possibility is that Harry is put in a situation where he sees the flaws in the wizarding society that Dumbledore and others represent. He might think that Voldemort's radical ideas, although extreme, could be a starting point for change. For example, the pure - blood and half - blood discrimination that exists in the wizarding world. Harry could believe that by working with Voldemort, he can bring about a more equal society, but of course, this would be a very complex and dangerous path to take.
Well, it could be that in this fanfiction, Harry discovers some long - hidden truth about the wizarding world that makes him realize Voldemort's motives aren't entirely baseless. Maybe there's a prophecy misinterpretation that leads Harry to think they can work together to fix the wizarding world's problems.
The Weasley family in general would be involved. Ginny, for instance, would be heartbroken seeing Harry on Voldemort's side. The family's loyalty to Harry would be put to the test, and they would be among the main characters as they try to either bring Harry back or fight against him if he goes too far with Voldemort.
A plot twist might be that when Harry goes to Voldemort, he discovers that Voldemort has a secret weakness. Maybe it's related to his family or some long - forgotten magic. Harry then has to decide whether to use this weakness against Voldemort or try to redeem him. This would add a whole new layer of complexity to the story. Another aspect could be that there are other forces at work, like a third - party wizard who is manipulating both Harry and Voldemort for their own gain, and Harry has to figure out who this is while dealing with Voldemort.
Perhaps in this fanfiction, Harry discovers some dark secret about the Order of the Phoenix or Dumbledore that makes him lose faith in the side he has always been on. This, combined with Voldemort's manipulation, could lead him to side with the Dark Lord. For example, if he finds out that Dumbledore has been using him all along without his full knowledge, he might be so disillusioned that he turns to Voldemort as an alternative.
Well, one way could be through some dark and misinterpreted magic. Maybe Harry stumbles upon an ancient spell that he thinks will defeat Voldemort once and for all but instead, it backfires and revives him. It could also be due to some sort of interference from a powerful dark object that Harry unknowingly uses in the wrong way.
The wizarding world might be in shock. People who have always seen Voldemort as the ultimate evil would be confused. They would have to re - evaluate their views on Voldemort. Harry, on the other hand, would be in a very strange situation. He might be distrusted by some of his friends who think he has made a deal with Voldemort.
One possible plot point could be the discovery of this relationship. Maybe Harry finds some hidden magical artifact or document that reveals the truth. Another could be the reaction of the wizarding world. Would they still see Harry as a hero or would they be suspicious of him? Also, Voldemort's reaction if he were to find out before his death could be interesting. He might try to use Harry for his own ends in a different way than in the original story.
One plot twist could be that Harry is actually a double agent. He pretends to be fully under Voldemort's control while secretly working against him. Another could be that Voldemort's training awakens a hidden power in Harry that is neither good nor bad, but something completely unexpected that changes the whole dynamic of the wizarding world.
Themes could include the blurred lines between good and evil. If Harry is a fan of Voldemort, it challenges the traditional view of their roles. Another theme might be the exploration of dark magic. Since Voldemort is associated with it, Harry's fascination could lead to a deeper look into the nature of dark magic.
Well, perhaps Pettigrew gets tired of living in fear under Voldemort. He might find a powerful magical artifact that has the potential to kill Voldemort. As he tries to steal it for himself, Voldemort discovers his plan. In the ensuing struggle, Pettigrew manages to turn the artifact against Voldemort and kills him. This would then lead to Pettigrew having to deal with the consequences, like facing the wrath of the Death Eaters who are still loyal to Voldemort.
One possible plotline could be Harry being held captive by Voldemort in a way that's like a pet. Voldemort might use Harry to show his power over the Boy Who Lived. Another could be that Harry has some sort of magic that makes Voldemort keep him as a pet instead of killing him immediately.