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.
If such a fanfiction existed, it would be a very dark exploration of power, coercion, and the blurring of the lines between good and evil. Harry, who represents good, would be in a situation that goes against all his values. It would also raise questions about the nature of the child - would it inherit Voldemort's evil or could it be redeemed? And how would the wizarding world react to such an abomination? There would be a lot of political and social unrest in the story, as people would be divided on how to deal with this situation.
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.
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.
At first, Harry would be conflicted. He has always been against Voldemort, but now being forced to be with him would make him question his own beliefs. His character could become more cynical as he is exposed to Voldemort's dark world.
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.