One possible plotline could be that Harry, after being abused, feels a sense of desperation and a lack of support from the good side. He might be lured by Voldemort's power and the false promises of a new order where he won't be abused. Voldemort could manipulate Harry by showing him a different way of life, perhaps one where he has more control and respect.
Well, it could start with Harry being constantly mistreated at Hogwarts or in the wizarding world. He then starts to question the values of the side that claims to be good. When Voldemort approaches him, Harry sees an opportunity to break free from his abusers. Maybe Voldemort offers to teach Harry dark magic to defend himself, and Harry, in his vulnerable state, starts to consider joining the dark side.
Perhaps Harry joins Voldemort because he believes he can change him from the inside. In this fanfiction, Draco could be the one who initially tries to stop Harry but then realizes that Harry's plan might actually work. They then form an unlikely alliance to bring down Voldemort's more evil plans while Harry is still pretending to be on Voldemort's side.
One possible plotline could be that Voldemort discovers Harry's abuse through his Death Eaters' spying. He decides to save Harry as he wants to turn Harry to his side and believes that by rescuing him, Harry will be more likely to join him. Another could be that Voldemort has a moment of self - reflection and realizes that the way Harry is being treated is wrong, so he steps in.
In some versions of this type of fanfiction, there could be a time - travel element. Harry travels back in time and ends up getting involved with a younger Voldemort. He sees potential in Voldemort that he believes can be redirected for good, but instead, he gets drawn into Voldemort's dark plans. As a result, he begins to work with Voldemort against the very people he once called friends and allies.
One possible plotline could be Harry initially being very distrustful of Voldemort, but as time goes on, he starts to see a different side of him. Voldemort, on the other hand, might be trying to use Harry for his own purposes at first but then develops a real bond. Another could be that Voldemort has to face the wizarding world's reaction to his newfound role as Harry's protector, which could lead to conflict. And Harry could grow to become a powerful ally to Voldemort in a new way, changing the power dynamics in the wizarding world.
In such fanfiction, a common plotline is that Harry's decision to join Voldemort creates a power shift. He might use his status as the 'Boy Who Lived' to undermine Dumbledore's reputation further. Since many in the wizarding world had put their faith in Harry, his joining Voldemort could lead to widespread panic and confusion. Harry could also start to recruit other disillusioned wizards to Voldemort's cause, using his own charm and the false narrative that Voldemort is actually a liberator of the wizarding world from the old, restrictive ways.
Maybe Harry joins Voldemort because he discovers some dark secrets about the so - called good side in the wizarding world. He might think that Voldemort's way of ruling could bring a new order that is more just in his eyes. However, he soon realizes that Voldemort's true nature is far more cruel than he expected, leading to a conflict within himself.
One possible plotline could be that Harry is tricked by Voldemort. Voldemort might use some powerful dark magic or false promises to make Harry believe that joining him is the right thing to do. For example, Voldemort could claim to have a way to end all the bloodshed in the wizarding world if Harry helps him.
Well, think about it. Harry's relationship with his friends is ruined. Hermione might try to convince him to come back to the good side, but Harry, feeling so betrayed by his past experiences, would resist. With Voldemort, Harry would be in a very strange position. Voldemort doesn't really trust anyone, but he would use Harry to his advantage. Harry, in turn, might start to see Voldemort as a sort of father figure, which is a very warped view considering Voldemort's nature. And the wizarding world as a whole would view Harry as a fallen hero, and there would be a lot of fear and hatred towards him.
Perhaps Voldemort discovers a long - lost family secret of Harry's that gives him leverage. For example, he might find out that there is a curse on Harry's family that only he can break, but only if Harry joins him. Or Voldemort could be using some form of mind - control device that was created by an ancient dark wizard, which he uses on Harry to make him a part of his ranks.
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.