I'm not entirely sure as it's a very specific fanfiction. It might be about Tsukune from Rosario + Vampire having some connection to a Nephilim, perhaps exploring new powers or a different origin story for him within the context of the Rosario + Vampire universe.
Well, start with the big fanfiction platforms. FanFiction.net is a well - known place where many fanfictions are hosted. Search for 'rosario vampire tsukune is a nephilim' in their search bar. Another option is Archive of Our Own (AO3). It has a very organized tagging system, so if you search using relevant tags like 'Rosario + Vampire', 'Tsukune', and 'Nephilim', you might be able to locate it. Also, some niche fanfiction forums dedicated to Rosario + Vampire might also have it, but those are a bit harder to find.
First, know the characters well. Tsukune is a complex character with his own growth arc. Make sure to stay true to his personality. Also, understand the world of Rosario Vampire, like the different monster species and their cultures. Then, come up with an interesting plot. It could be a new adventure for Tsukune or a different take on his relationships.
Tsukune's vampire form in fanfiction can vary greatly. Some authors might focus on the emotional aspect of him dealing with this new side of himself. He could struggle with the urge to feed on blood, or he might embrace it as a part of his new identity. Additionally, his relationships with other characters, especially Moka, can be significantly affected by his vampire form.
A possible fanfiction could focus on a love triangle between Kurumu, Tsukune and another character. Kurumu is deeply in love with Tsukune, but Tsukune is conflicted as he has feelings for Kurumu but also has responsibilities towards others. This would lead to a lot of emotional drama and interesting character interactions.
One great Tsukune and Kurumu fanfiction could be a story where they go on a magical adventure together outside of the school. They might face some new and powerful enemies that test their relationship and skills.
Fanfiction could also center around Mizore and Tsukune training together. Since they both have different powers, they could learn from each other. Tsukune could teach Mizore about human resilience and determination, while Mizore could show Tsukune how to better control his powers, especially when it comes to the more vampiric aspects. Their training sessions could be filled with both comical and intense moments.
One possible fanfiction could be about Mizore and Tsukune going on an adventure together in the world of Rosario Vampire. Maybe they have to face a new kind of supernatural threat that has emerged in the school. They could use their unique abilities, Mizore's ice powers and Tsukune's growing vampire - human hybrid powers, to combat this threat and protect their friends.
One possible fanfiction could be about Nekonome and Tsukune going on a secret adventure in the school grounds. Maybe they discover a hidden passage that leads to a place full of magical artifacts. Nekonome's feline instincts could help them navigate through the dangerous parts while Tsukune's growing powers could be used to protect them from any threats.
One possible fanfiction could be about Akasha seeing Tsukune's unique nature among humans and vampires. Maybe she decides to test him in various ways, putting him in situations that make him question his own beliefs and values. Tsukune, in turn, could be both intrigued and a bit frightened by Akasha.