One popular 'Harry is Voldemort's Horcrux' fanfiction is 'The Horcrux Within'. It delves deep into how Harry's new status as a Horcrux affects his relationships with his friends, especially Hermione and Ron. It also shows his internal struggle as he tries to come to terms with this dark connection to Voldemort.
His reaction would likely be very different. At first, he might be angry that he didn't know this before. But then, he could start to see opportunities. He may think that if he can somehow control Harry as a Horcrux, he can gain more power. So, he might change his strategy from simply trying to murder Harry to trying to subdue him in a different way.
Well, it depends on how the author writes it. Maybe Voldemort would initially be in disbelief. But once he accepts it, he could start to manipulate Harry in different ways. He might think that by controlling Harry, he can control the Horcrux within him. This could create some really interesting plot twists where Harry has to resist Voldemort's attempts at a much deeper level than just physical attacks.
Well, a major aspect is the impact on the overall story arc. In these fanfictions, the fight against Voldemort takes on a whole new meaning. Harry being a Horcrux means that his very existence is intertwined with Voldemort's. There's also the element of secrecy and revelation. How and when the other characters find out about Harry being a Horcrux can be a crucial part of the plot. And, of course, there's the magical mechanics of what it means to be a Horcrux. How does it affect Harry's magic? How can it be destroyed without killing Harry? These are all things that are often explored.
If Harry were to create a Horcrux in fanfiction, it would likely lead to a great corruption of his soul. Horcruxes are dark magic, and the act goes against all that Harry stands for in the original series. He might start to show signs of increased aggression, a detachment from his friends, and a growing attraction to the Dark Arts.
Well, it could be that Dumbledore, in some fanfic versions, figures it out first and then reveals it to Harry. Dumbledore is often portrayed as having a deep understanding of magic. He might have noticed some strange magical patterns or behaviors related to Harry that led him to suspect and then confirm that Harry was a Horcrux. And then he would tell Harry about it, which is how Harry comes to know.
Perhaps his father's old wand. It has a connection to his family and his heritage. Since it's a wand, it already has a certain amount of magical energy, so it could potentially be a suitable vessel for a Horcrux. And it would be a very interesting twist on the concept of a Horcrux as wands are not a typical choice.
One consequence could be that his connection to Voldemort becomes even stronger. This might lead to him being able to sense Voldemort's actions and thoughts more clearly, but also make him more vulnerable to Voldemort's attacks on his mind. Since the Horcrux is a part of Voldemort's soul, it could also start to influence Harry's personality, making him more aggressive or moody at times.
One consequence could be that Harry feels an internal struggle. Since he has a part of Voldemort's soul in him, he might have to constantly fight against the dark impulses that come with it. For example, he may find himself having more aggressive thoughts or being more easily influenced by dark magic in certain situations.
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.