In the 'harry is adopted by voldemort fanfiction', it could start with Voldemort seeing potential in Harry that no one else does. He takes Harry in and raises him in a very different way than the Dursleys did. Harry might be exposed to a world full of dark magic and Voldemort's grand plans. But as he gets older, Harry's sense of right and wrong, which he got from his parents, starts to surface. He might then try to find a way to use the knowledge he gained from Voldemort to defeat him, while also struggling with the emotional bond that has developed between them, if any.
Well, in this fanfiction, perhaps Voldemort adopts Harry because he realizes Harry has some unique power that he wants to control. As Harry grows up, he starts to understand Voldemort's motives and tries to find a way to break free. He could also form unexpected alliances with some of Voldemort's followers who have a change of heart.
One possible plot could be that Harry is somehow tricked into believing Voldemort's cause is just. Maybe Voldemort uses some powerful magic to distort Harry's perception of good and evil. So Harry starts to help Voldemort in his quest for power, thinking he is doing the right thing.
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.
A really interesting plot twist might be that Voldemort is being controlled by an even darker force, and Harry has to figure out a way to break that control while being held captive. Or perhaps Harry's friends, thinking he is dead, accidentally start a chain of events that end up helping Harry escape in a very unexpected way. For example, Hermione might cast a spell that was supposed to be for protection but ends up creating a portal right in Voldemort's lair.
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.
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.
Perhaps Dumbledore is being forced in this fanfiction. There could be some dark magic or powerful dark wizard threatening Hogwarts or the wizarding world. Dumbledore, in a desperate attempt to save everyone else, gives Harry to Voldemort as a sacrifice. But Harry, being the hero he is, could find a way to turn the situation around and defeat Voldemort from within his lair.
The plot might go like this. Harry is adopted by Voldemort and is made to believe that the wizarding world outside is full of enemies. But as he gets older, he discovers the truth about his parents' death and Voldemort's true nature. He then has to struggle with his loyalty to Voldemort who raised him and his sense of justice. He could use his unique position to gather intelligence about Voldemort's plans and eventually turn against him to save the wizarding world.
One possible plot could be that they are forced to be partners to save the wizarding world from an even greater threat. Maybe an ancient evil that neither of them can face alone. So, they have to put aside their differences and work together, which would be a huge change from their usual enmity.
In a 'harry joins voldemort fanfiction', perhaps Harry discovers a dark secret about the wizarding world that makes him think Voldemort was right all along. It could be something about the Ministry of Magic being corrupt and Voldemort's way of ruling, although extreme, is the only solution. So he joins Voldemort to 'fix' the wizarding world.
In such a fanfiction, Harry might be raised with a very different set of values. Voldemort could try to mold Harry into his own image, perhaps teaching him dark magic. But Harry might also secretly hold on to his own sense of good. For example, he could use the knowledge Voldemort gives him to ultimately turn against him.