In such a fanfiction, Harry might be used as a bait to lure his friends into a trap. Voldemort could try to extract information from Harry about the Order of the Phoenix or Dumbledore's plans. Maybe he would also attempt to turn Harry to the dark side by showing him the so - called 'power' of the Dark Arts.
The idea of Voldemort getting Harry pregnant goes against the original nature and themes of the Harry Potter series. J.K. Rowling's work is mainly about good vs. evil, friendship, and the fight against dark magic. So, while there might be some extremely non - canonical and out - there fanfiction on this very strange and inappropriate topic, it is not something that is in line with the spirit of the original story.
The Death Eaters are an obvious choice. They've always been after Harry, so they could easily be the ones to kidnap him. They'd want to take him to Voldemort to prove their loyalty and also to finally get rid of Harry for good.
Well, perhaps Voldemort's followers could kidnap Harry to use him as a bargaining chip. They might hold him in an old, dark wizarding house, guarded by dark magic. Harry would then try to escape using his own magic skills and the help of some unexpected allies like house - elves who secretly oppose Voldemort.
If Voldemort gets custody of Harry in fanfiction, it would be a huge deviation from the original story. Harry would probably be exposed to a lot of dark and cruel things. Voldemort could try to break Harry's spirit and make him abandon his values. There might be a struggle within Harry between the good he has been taught and the evil that Voldemort represents. And the wizarding world would be in even more chaos as Voldemort would have more control over the one person who was seen as the hope to defeat him.
A really interesting plot twist might be that Voldemort is being controlled by an even darker force, and Harry has to figure out a way to break that control while being held captive. Or perhaps Harry's friends, thinking he is dead, accidentally start a chain of events that end up helping Harry escape in a very unexpected way. For example, Hermione might cast a spell that was supposed to be for protection but ends up creating a portal right in Voldemort's lair.
One popular 'Harry Kidnapped by Voldemort' fanfiction might be 'The Darkest Captivity'. In this fic, the author explores how Harry endures the situation in Voldemort's lair, his internal struggles and the unexpected alliances he forms with some of Voldemort's followers. Another one could be 'Harry's Ordeal in the Dark Lord's Grasp' which delves into the psychological impact on Harry during his captivity.
First of all, Voldemort would probably be gloating over his capture of Harry. Harry's friends would then start to search for him. Hermione might use her vast knowledge of magic and research to figure out where Voldemort could be hiding Harry. Ron would be the more impulsive one, wanting to rush in and save Harry right away. As the story progresses, there could be a final showdown between Harry and Voldemort where Harry, despite being kidnapped, manages to turn the tables on Voldemort.
In this kind of fanfiction, one possible scenario is that Harry's whole view of the wizarding world would be warped. Voldemort might try to mold Harry into his own image. He could teach Harry dark magic instead of the traditional magic taught at Hogwarts. Harry might grow up with a completely different set of values and powers, which would surely change the whole dynamic of the fight against good and evil in the wizarding world.
In some fanfictions, if Harry Potter gets broken by Voldemort, it could mean his spirit is crushed. Maybe he loses his will to fight and his friends have to find a way to bring him back. It might involve them using powerful magic or memories to reignite his courage.