It changes the dynamic in many ways. For starters, the relationship between the different magical creatures and humans would be affected. Creatures that were once on the side of good might start to reevaluate their alliances. The pure - blood families that were on the fence about Voldemort would now be more likely to side with him, seeing that Fem Harry has joined. The magical economy would also take a hit. With the threat of Voldemort growing stronger with Fem Harry by his side, businesses would struggle as people are more focused on survival. And the educational system at Hogwarts would be overhauled. The curriculum might start to include more teachings on dark magic prevention, and there would be a stricter divide between students who support Fem Harry's actions and those who don't.
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.
The Death Eaters would initially be overjoyed. They would think that with Harry on their side, they are invincible. But some of them might also be suspicious of Harry's motives, fearing that he is a double - agent. As for the general wizarding population, they would be terrified. Harry was seen as their hope, and now that he has joined Voldemort, they would feel completely hopeless and defenseless.
The wizarding world would be in shock. People who once saw Harry as their savior would feel betrayed. The Order of the Phoenix would be in disarray, not knowing whether to try to rescue Harry or consider him an enemy. Hogwarts would also be in turmoil, with students and teachers alike not believing that Harry could turn to the dark side.
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.
The wizarding world would likely be plunged into a state of despair. Harry was seen as the 'Chosen One' to defeat Voldemort, so without him, there would be a sense of hopelessness. Voldemort would probably have free rein to impose his dark rule without much opposition at first.
When Harry Potter joins the dark side in fanfiction, the wizarding world undergoes a major transformation. Fear would spread among the good wizards and witches. Dark magic would likely become more prevalent as Harry's influence could encourage others to explore it. The balance between good and evil would be severely disrupted. The families of Harry's friends would be in danger, and there might be a split within the Hogwarts faculty on how to deal with him. Also, the centaurs and other magical beings might get involved, either siding with the dark side because of Harry or trying to stop him to protect their own interests.
The wizarding world would be in chaos. Harry, being Voldemort's right - hand man, would be a huge threat. He knows many of the Order of the Phoenix's secrets. Hogwarts would no longer be a safe haven. The students would be in constant fear as Harry could use his inside knowledge to help Voldemort attack the school. The Ministry of Magic would also be in a difficult position as they had once relied on Harry to be their savior.
If Voldemort won the war in fanfiction, the wizarding world would experience a total transformation. First, the Ministry of Magic would be under his complete control. His Death Eaters would be in charge of all the important positions. The concept of justice would be warped, and any form of opposition would be brutally suppressed. The wizarding economy would likely suffer too as Voldemort's focus would be on power and domination rather than development. Also, the relationships between different magical creatures would be disrupted as Voldemort has no respect for any form of life other than his own pure - blood ideology.
One interesting plot twist could be that Fem Harry's presence among Voldemort's ranks causes internal conflict. Some Death Eaters might be distrustful of her because she was once the 'Boy Who Lived' in a female form. Another twist could be that Fem Harry starts to influence Voldemort's ideology in a way he didn't expect. Maybe she makes him question his pure - blood supremacy by showing that her Muggle - born friends were as powerful as any pure - blood wizard. Also, it could be that her joining was part of a long - con plan to bring down Voldemort from the inside, and she gradually starts to turn his own followers against him.
Well, it could be that her joining Voldemort causes a huge shift in the balance of power. The wizarding community would be in chaos as they try to figure out how to deal with this new threat. Dark Fem Harry might also start to influence Voldemort in unexpected ways. Maybe she convinces him to use more psychological warfare rather than just brute force. And she could be the one to find and destroy some of the remaining Horcruxes of other wizards, believing that it would make Voldemort even more powerful and thus her position more secure.
There would be a great deal of shock and confusion. Many would start to question everything they had believed in. Families would be divided. Some might think that Harry had been corrupted by Voldemort's dark magic and try to find a way to reverse it. The Ministry of Magic would be in a panic. They would try to come up with new strategies to deal with this new threat. Teachers at Hogwarts would be worried about the safety of their students. And the Death Eaters, while initially happy to have Harry on their side, would also be suspicious of his motives and loyalty.