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.
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.
She would be terrified at first, but quickly gather her wits. Hermione is very intelligent, so she would likely start observing her surroundings, looking for any possible means of escape or a way to send a signal to her friends.
In one possible plot, Voldemort might kidnap Hermione to use her knowledge of magic against Harry. He could try to force her to reveal secrets about Dumbledore's plans or the location of certain powerful magical artifacts. Hermione, being strong - willed, would resist and try to outwit Voldemort using her intelligence.
Harry often becomes more conflicted. He has always seen Voldemort as the ultimate evil, but now being mates, he has to re - evaluate his views. He might start to see the humanity in Voldemort, which makes him question his own beliefs and actions in the fight against the Dark Lord. For example, he may hesitate in battles more than before.
Often in fanfiction, their relationship can change drastically. They could end up as allies, which would require both of them to change. Harry would have to see past Voldemort's evil deeds, and Voldemort would have to let go of his hatred for Harry. This exploration of character change makes fanfiction so interesting.
Maybe Harry uses some powerful and unexpected spells. For example, he could combine the Patronus Charm with a blasting spell. The Patronus might distract Voldemort while the blasting spell hits him hard.
In this fanfiction, the relationship between Harry and Voldemort would be full of contradictions. Voldemort adopts Harry for his own purposes, but Harry has his own will. In the beginning, Harry may be forced to follow Voldemort's orders out of fear. As time passes, he might start to question Voldemort's actions and ideology. Voldemort, seeing Harry's growing independence, could become more controlling. This could lead to a cycle of rebellion and suppression in their relationship. Harry may also find some common ground with Voldemort in their shared loneliness or desire for power, which would add more layers to their complex relationship.