Another option is to check out Reddit. There are subreddits dedicated to different fandoms, and you can post in those relevant subreddits asking for this specific fanfiction. People in the community might be able to point you in the right direction. Also, browsing through the existing posts might lead you to what you want.
I'm not sure specifically about 'oc has true longinus vs gilgamesh' fanfiction. But you could try searching on fanfiction - specific websites like Archive of Our Own or FanFiction.net. They usually have a wide variety of fan - made stories, and you might be able to find something related to that particular scenario by using relevant tags and keywords in your search.
One possibility could be that the True Longinus gives Harry an unexpected power boost. He might be able to use it to defend Hogwarts in a more powerful way against Voldemort. For example, it could enhance his spells and make them more effective against the Dark Arts.
In a fanfiction, the True Longinus might be a gift from a mysterious figure. This figure could be an ancient wizard who has been observing Harry. The True Longinus then becomes a tool for Harry to combat the growing darkness in the wizarding world. It could have unique magical properties that are different from the wands and magic we know in the Harry Potter series. For instance, it might have the ability to nullify dark magic spells completely, which would be a huge advantage for Harry.
The plot could involve the discovery of the True Longinus in the wizarding world. Harry, being the curious and brave wizard he is, decides to study it. As he delves deeper, he realizes that it has a connection to an ancient magic that was long forgotten. This knowledge helps him not only in his fight against Voldemort but also in understanding the true nature of magic. The True Longinus could also attract the attention of other dark forces, leading to new and more dangerous enemies for Harry to face.
Another possibility is that a group of dark wizards could be the main antagonists. They see the True Longinus as a tool to bring about a new era of darkness. They band together to try and take it from Harry, and they use all kinds of dark magic and underhanded tactics. They might even try to turn some of Harry's friends against him in their quest to get the True Longinus.
In such a fanfiction, having the OC with multiple mates could make the plot really interesting. It might involve juggling different relationships, dealing with jealousy, or exploring unique power dynamics. But it also might risk overcomplicating the story if not handled well.
It could be an original character created just for the fanfiction. Maybe a mysterious ninja woman who crossed paths with Kakashi during a mission and they had a brief but intense relationship.
Often, there are original characters created specifically for the fanfiction. These OCs are the main focus with their dark quirks. There might also be some canon characters from the original work that the fanfiction is based on. They could be friends, foes or mentors to the OC. For example, if it's a fanfiction based on a superhero story, there could be a well - known hero who tries to help the OC control their dark quirk.
First, study the lore of True Blood. Know the rules about vampires, shapeshifters, and other supernatural beings. For example, if your OC is a vampire, understand how they feed, their weaknesses, and their powers. Then, give your OC a unique backstory. Maybe they were a noble in their human life, which affects their vampire behavior. Also, think about their personality. Are they kind - hearted like Godric or more ruthless? Their personality should be consistent throughout the story.