One common theme is identity struggle. Harry has to figure out who he is in relation to being Voldemort's son. Another theme could be redemption. Maybe Harry tries to make up for his father's evil deeds. And there's often the theme of power. Whether Harry has the same kind of power as Voldemort or a different, more controlled power.
In this fanfiction, it could be that Voldemort has a premonition. He sees that if Harry Potter dies, it will lead to a series of disasters that will also destroy his own plans. So, in order to preserve the balance or to keep his long - term goals intact, he saves Harry. For example, if Harry's death causes an uncontrollable chaos in the wizarding world, Voldemort's rule over the dark forces might be disrupted. So he steps in to save Harry.
There are a lot of Harry Potter doujinshi that you can refer to:
1 Harry Potter and Defense Against the Dark Arts: This is a very popular Harry Potter fanfiction about the confrontation between Harry Potter and Voldemort in Defense Against the Dark Arts.
2 Harry Potter and the Goblet of Fire: This is another very popular Harry Potter fanfiction about the adventures of Harry Potter and Voldemort in the Goblet of Fire.
3 Harry Potter and the Prisoner of Azkaban: This is a darker Harry Potter fanfiction book that recounts Harry Potter's experiences in the Prisoner of Azkaban, including his confrontation with Voldemort.
4 Harry Potter and the Order of the Phoenix: This is another very popular Harry Potter fanfiction about Harry Potter's fight against Voldemort in the Order of the Phoenix to protect Hogwarts.
5 Harry Potter and the Half-Blood Prince: This is another heartwarming Harry Potter fanfiction about the reconciliation between Harry Potter and Voldemort in the Half-Blood Prince.
Perhaps in this fanfic, there has been some major plot twist that makes Harry believe that bowing to Voldemort is the only way to save his friends or the wizarding world. For example, Voldemort may have threatened to kill all of Harry's loved ones if he doesn't submit, and Harry is sacrificing his pride for the greater good.
If Harry Potter was Voldemort's son in fanfiction, it would completely change the story. Firstly, the relationship between Harry and Voldemort would be extremely complex. Instead of being arch - enemies, there would be a strange family bond. Harry might struggle with his identity more. He would have to deal with the fact that his father is the most evil wizard. The Hogwarts faculty might also treat him differently. They would be more suspicious of him and his motives, fearing that he might inherit Voldemort's evil ways. Also, his friends might have a hard time accepting this new truth about him.
Well, it changes it drastically. In the original, Harry is the hero against Voldemort. But in this fanfiction, the dynamic is completely different. Harry's abuse gives him a different background and the revelation of his relation to Voldemort makes him an outcast in a new way.
A really good one is 'Voldemort's Redemption'. This fanfiction flips the script a bit. It explores the possibility of Voldemort being redeemed. It's a fascinating read as it shows how Harry might react to a Voldemort who is trying to change. It also has some great descriptions of the magic and the Hogwarts environment, making it feel like a natural extension of the Potterverse.
Harry could become more powerful in a way that scares his friends. Voldemort's training might focus on the Dark Arts, which are powerful but dangerous. This could change Harry's magic style. He may become more solitary as his new skills and knowledge set him apart from his Hogwarts friends who follow a more traditional path of magic.
The Weasley family in general would be involved. Ginny, for instance, would be heartbroken seeing Harry on Voldemort's side. The family's loyalty to Harry would be put to the test, and they would be among the main characters as they try to either bring Harry back or fight against him if he goes too far with Voldemort.
He might have enhanced Parseltongue abilities. Since Voldemort was a master of Parseltongue, Harry could have an even greater proficiency in communicating with snakes. He could also potentially have a stronger connection to dark magic, but be able to resist it more powerfully than others due to his upbringing with the Dursleys and his time at Hogwarts. His Patronus could be something more complex, perhaps a combination of a stag (from his father James) and a snake (from Voldemort), representing his dual nature.