Well, in some 'Dark Harry joins Voldemort' fanfictions, Harry could be driven by revenge. Maybe he blames Dumbledore or the wizarding world for the death of his parents in a different way. He sees Voldemort as an ally who can help him take revenge. Then, as he joins Voldemort, he might start to question the very nature of good and evil. And perhaps he'll find out that Voldemort has a different side to him that he didn't expect.
One possible plot could be that Harry, influenced by his darker side, joins Voldemort to gain power. He might start to learn the Dark Arts from Voldemort directly. Another plot could be that Harry believes Voldemort has some secrets about his family that he can only learn by getting close to him. Also, it could be that Harry is tired of the Order's way and thinks Voldemort's methods are more efficient in achieving his own goals.
One interesting plot could be how Harry and Voldemort initially react to the betrothal. Maybe Harry is completely shocked and disgusted at first, while Voldemort sees it as a way to further control the wizarding world through Harry. Another plot point might be how the wizarding community reacts. Some might be horrified, thinking it's a dark and twisted union, while others might see it as a potential way to end the long - standing conflict between good and evil.
Another plot could be the development of the relationship between Harry and Voldemort. At first, they would be extremely hostile towards each other. But as time passes, they might start to understand each other in unexpected ways. For example, Harry could teach Voldemort about love and friendship, concepts that Voldemort has long discarded, which could potentially change Voldemort's dark path.
One interesting plot could be that they are mates due to a powerful ancient magic that was long forgotten. This magic forces them to understand each other's pasts and they start to question their long - held enmity. For example, they might discover that they both had difficult childhoods which led them to their current paths, and this newfound understanding makes them consider a different future together.
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.
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.
In some of these fanfictions, the plot might involve Harry and Voldemort having a secret family connection. For example, they could be distant cousins through some convoluted wizarding family tree. This changes how they view each other and the conflict between them.
In some stories, their friendship could start from a time - travel accident. Harry gets sent back in time and ends up befriending a young Voldemort. He tries to influence Voldemort's path, and they form an unlikely bond. Harry might introduce Voldemort to different values and ideas that were not present in Voldemort's original upbringing, which causes a major shift in the future of the wizarding world.
Perhaps in the fanfiction, Harry gives up because he becomes disillusioned with the wizarding world. He may see all the corruption and prejudice within it and decides that Voldemort's way of ruling might not be much worse. This could lead to a story where Harry actually joins Voldemort's side, not as a slave but as an 'advisor' of sorts, trying to change Voldemort from the inside. However, this is a very dangerous and morally ambiguous path for Harry to take.
In some fanfictions, young Harry might kill Voldemort by accident. For example, he could be exploring an old, forgotten chamber in Hogwarts and stumble upon a powerful magical artifact. When he touches it, it releases a burst of magic that targets Voldemort's main soul fragment, wherever he is. This causes Voldemort to perish instantly, leaving Harry to deal with the aftermath and the shock of what he has unknowingly done.
One interesting plot could be that they are distant relatives through some ancient wizarding bloodline. Maybe there was a curse placed on the family long ago that led to their enmity.