The book was called HP: Back to Future.
Harry might use his wits and the environment around him while in captivity. For example, if he's held in a place with magical artifacts, he could use them to his advantage. He could break the chains that bind him with a spell - enhanced object and then make his way to Voldemort and defeat him.
Hermione is often a popular character. She might play a role in helping Harry understand the implications of Voldemort's reincarnation, using her vast knowledge of magic and history.
😋I recommend you to read HP Doujin: Reversal. This is a fan-fiction novel, the main character is Draco Malfoy, not the Harry Trio, not Gryffindor, not Old Deng, and definitely not Voldemort! The female protagonist had transmigrated to reverse the direction of some of the characters in the original novel. It was a very good Harry Potter Doujin novel. I hope you like my recommendation, Mwah ~😗
It could be Dumbledore from beyond the grave. He might find a way to reach out and summon Harry, believing that only Harry has the potential to truly defeat Voldemort in this new, dire situation.
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.
One great 'Harry Potter x Voldemort fanfiction' is 'The Redemption of Tom Riddle'. It explores an alternate universe where Harry tries to understand Voldemort's past and finds a way to redeem him. Another interesting one is 'A Different Bond'. It focuses on a magical bond that forms between Harry and Voldemort, changing the course of their enmity. And 'The Soul's Connection' delves into how their souls, both having a piece of the other, interact in unexpected ways.
There are some out there. You can start by looking on fanfiction websites like Archive of Our Own. Some fanfics might explore the complex relationship between Voldemort and Harry through the connection with Quirrell. However, the quality can vary greatly, so it might take some time to find one that really suits your taste.
One great fanfic in this category is 'The Dark Alliance'. It explores a complex relationship between Voldemort and a darker version of Harry Potter. The story is filled with intense power struggles and moral dilemmas.
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.