When Harry is a Horcrux in this fanfic and Sirius discovers it, there are numerous implications. Firstly, Harry's very soul is at stake. His identity as a Horcrux means that a part of Voldemort's soul resides within him. This could lead to internal struggles for Harry, both morally and magically. Morally, he might feel tainted by this connection. Magically, it could mean that his powers are influenced by Voldemort's dark magic. Moreover, the wizarding world's perception of Harry could change if this knowledge were to spread. Some might see him as a threat or a tool of Voldemort. Sirius would have to not only deal with the immediate danger to Harry but also with the long - term implications for Harry's place in the wizarding world and his own sense of self.
One implication is that Harry's life is in even more danger. Voldemort might be more determined to keep him alive until he can fully use Harry for his own evil purposes.
Sirius might be shocked. He'd probably feel a mix of horror and protectiveness. He'd be horrified that Voldemort has used Harry in such a way and would be even more determined to keep Harry safe from Voldemort's clutches.
Well, Sirius is a very protective character. When he discovers that Harry is a Horcrux in the fanfic, he could initially be in denial. But once he accepts it, he'd likely start looking for ways to either protect Harry from Voldemort or find a way to remove the Horcrux part from Harry. He might also feel angry that Harry has been put in such a dangerous situation.
One possible consequence is that Harry becomes an even bigger target for Voldemort. Since a Horcrux is important to Voldemort's immortality, he'd be more determined to get to Harry. Also, Harry might start to experience some strange effects on his soul or magic due to being a Horcrux.
One implication is a deeper connection between Harry and Voldemort. Their fates are even more intertwined than in the original story.
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.
In this kind of fanfiction, it often shows Ginny's shock and the complex emotions she might have. She might be worried about Harry's fate, as being a Horcrux means he is tied to Voldemort in a very dangerous way. This discovery could lead to her trying to find a way to break the Horcrux connection without harming Harry.
The theme of self - discovery for Ginny is often seen. As she delves deeper into the situation of Harry being a Horcrux, she might discover new aspects of herself, like her own strength in dealing with such a complex and dangerous situation. And there's also the theme of hope, that despite the seemingly impossible situation, there is always a way to save Harry and defeat 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.
In a 'harry potter and the last horcrux fanfic', it may center around the significance of the last Horcrux. It could depict the emotional and mental state of Harry as he approaches the end of his long journey. There could be unexpected twists, like the Horcrux being hidden in a place no one suspected. It might also expand on the lore of Horcruxes, for example, by showing different ways to detect or destroy them that were not mentioned in the original series.
One thing to keep in mind when looking for such a fanfic is that it might be a bit hard to find as it's a rather specific and dark concept. Some fanfic writers might be hesitant to write about such extreme torture scenarios. You could also check fanfic communities or forums dedicated to Harry Potter. Post a request there asking if anyone knows of a fanfic with this plot. Maybe someone has come across it or has even written one themselves.