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.
Yes, there are some. You can try searching on fanfiction websites like FanFiction.net or Archive of Our Own. Some stories might explore an alternate universe where Harry joins Voldemort for various reasons, such as being manipulated or having a different view of the wizarding world's power structure.
One possible plotline could be that Harry is tricked by Voldemort. Voldemort might use some powerful dark magic or false promises to make Harry believe that joining him is the right thing to do. For example, Voldemort could claim to have a way to end all the bloodshed in the wizarding world if Harry helps him.
In a 'Harry and Sirius join Voldemort' fanfiction, it might start with Voldemort making a very tempting offer. For Harry, it could be something related to saving his friends or family. Sirius might be lured by the idea of protecting Harry in a different way. However, once they join, they would face a moral dilemma. They would have to pretend to be on Voldemort's side while still holding onto their own values. And there would be a lot of internal conflict as they see the evil deeds happening around them.
This is a very strange and dark concept for fanfiction. In a possible plot, perhaps they are under some powerful mind - controlling spell. Maybe Voldemort has found a way to use their skills and knowledge for his own evil ends. For example, Harry's connection to the Deathly Hallows could be exploited, and Ginny's powerful magic, which was shown in the books, could be used to carry out Voldemort's orders against their will.
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.
Harry could be used as a symbol of power. Voldemort might make Harry a sort of figurehead to show that even the 'Chosen One' has joined him, demoralizing the resistance. Hermione, on the other hand, could be forced to use her knowledge for Voldemort's dark magic research, but secretly she could be sabotaging it.
One possible plot could be that they are forced to join Voldemort under some sort of dark magic compulsion. Maybe Voldemort has a new powerful spell that he uses on them to make them his servants. Then they have to struggle with their own moral compasses while doing his bidding, and secretly try to find a way to break free and defeat him from the inside.