There are indeed such fanfictions. Some explore really interesting concepts within this odd scenario. For example, in some stories, the forced marriage is a part of a dark plot by Voldemort to gain more power over the wizarding world through Harry. Others might focus on how Harry copes with this situation and tries to find a way out while also dealing with his complex feelings towards Voldemort. It's a very unique concept that can lead to some very creative storytelling.
Yes, there are some fanfictions with this rather strange premise. You can often find them on various fanfiction websites like Archive of Our Own (AO3). Just search for the relevant tags.
One plotline could be that Voldemort wants to use the marriage to gain control over Harry's special powers. Another could be that it's part of a prophecy that neither of them can avoid. Or maybe it's a political move in the wizarding world to end the war between the two sides.
One possible plot development could be that Harry tries to find a way to escape the marriage. She might use her magical skills and the help of her friends to break free from Voldemort's control. Another could be that she pretends to go along with the marriage while secretly planning to undermine Voldemort from within.
Finding such a specific fanfiction can be a bit of a challenge. Some fanfiction communities on platforms like Tumblr might also be a good place to ask. There could be fans who have come across this type of story and can recommend it. Another option is to check out some Harry Potter fanfiction forums. People there are often very helpful in sharing their favorite reads.
The theme of identity is often present in 'harry marry voldemort fanfiction'. Both Harry and Voldemort have to re - evaluate who they are in the context of their relationship. Another common theme is the struggle against society's norms. Since their relationship is so unorthodox, they face a lot of opposition from the wizarding world. This struggle adds depth to the story and shows how their love can be strong enough to withstand external pressures.
I'm not aware of such a fanfiction existing. The idea is rather dark and goes against the general themes of the original 'Harry Potter' series. The series is mainly about good vs. evil, and this concept seems very out of place.
One reason could be for the sake of exploring different relationship dynamics. Fans want to see how Harry would react in a situation that is completely out of his control, like a forced marriage. It adds a new layer of drama to his character.
One reason could be some sort of ancient magical law or prophecy. Maybe there's a hidden clause that says for the greater good of the wizarding world, they need to be married. It could be related to some power or protection that their union might bring.