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.
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.
In these fanfictions, Harry's character transformation is quite drastic. He loses his sense of morality. His loyalty, which was once towards his friends and the good side, now shifts to Voldemort. He becomes more power - hungry, willing to do whatever it takes to gain more power, including using dark magic without hesitation. He also becomes more secretive and manipulative, using his former relationships to further Voldemort's goals.
Her character would likely become more ruthless. Since she's joining Voldemort, she'd have to adapt to his ways of using dark magic without hesitation. For example, she might no longer have the moral qualms about using the Unforgivable Curses.
Voldemort and Harry's relationship would be very strange. Voldemort might not trust Harry at first, even though he has given up. He would constantly be on guard, thinking it's a trick. And Harry, on the other hand, would have to navigate living in the Death Eaters' world. The other Death Eaters would also be suspicious of Harry, some might even try to kill him to prove their loyalty to Voldemort.
Often, Voldemort starts as this purely evil figure. But when he adopts Harry, he begins to show some unexpected softness. For example, he might start to protect Harry from other threats, which is very different from his usual self.
When Fem Harry joins Voldemort, it completely flips the power balance. The Ministry of Magic, which was already in a tenuous position, would be even more unstable. They would be unsure how to deal with a situation where the once - hero is now on the side of the dark lord. The Hogwarts students would be divided. Some might be inspired by Fem Harry's actions, thinking there's more to the story, while others would be heartbroken. This would also lead to a change in the way magic is perceived. Dark magic might start to seem more appealing to some, and light magic less so, creating a moral gray area in the wizarding world.
Yes, there are some fanfictions with this concept. It's an interesting take on the Harry Potter universe as it flips the typical relationship between Harry and Voldemort. In these fanfictions, the story often explores how Voldemort's actions change when he takes on the role of an adoptive figure to Harry, who has been abused. It can lead to some really unique character developments and plot twists.
The fact that Harry is abused gives a whole new depth to the story. It's not just about a boy joining a dark lord for no reason. His abuse makes him more susceptible to Voldemort's influence. It also adds a layer of tragedy to the 'romance' aspect. Instead of a normal, healthy relationship, it becomes a relationship born out of pain and disillusionment. Harry's abuse could lead to a cycle of him trying to find validation and love in all the wrong places, with Voldemort being that wrong place in this fan fiction.
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 this fan fiction, the plot could involve Harry's abuse leading him to question his previous beliefs. Voldemort, being a powerful figure, might take advantage of Harry's vulnerability. The 'romance' could be a complex web of power dynamics and emotional manipulation. For example, Voldemort might use his charm and knowledge to draw Harry closer, and Harry, in his state of confusion and hurt from the abuse, might be swayed into believing that there is something more to their relationship than just power - seeking.