One of the main themes is isolation. The OC, being a descendant of leviathan, might be different from others and feel alone. Also, the theme of discovery is prevalent. They are constantly uncovering more about their leviathan heritage, like hidden powers or ancient secrets. And of course, there's the theme of conflict, whether it's internal or against those who fear or want to use their powers.
Power and responsibility are common themes. Since the OC is a descendant of leviathan, they often have to deal with new - found powers and figure out how to use them responsibly. Another theme is identity. They may struggle with who they are in relation to this powerful and often mysterious heritage.
There's a 'oc descendant of leviathan fanfic' that I've heard about called 'Descendant's Destiny'. In this fanfic, the OC is on a journey to discover the true meaning of their connection to the leviathan. It has some exciting action scenes and a great deal of character development. The author does a good job of exploring the concept of being a descendant in a unique and engaging way.
I'm not entirely sure specifically as it could vary greatly. It might be about an original character (oc) who is a descendant in a story based on the Leviathan concept within a fan - created fictional work.
You could try searching on popular fanfic websites like Archive of Our Own or FanFiction.net.
One common theme is friendship. Many fanfics explore the bond that forms between Percy and Leviathan. For example, they might start as enemies but gradually become friends as they learn more about each other.
One common theme is power and its implications. Since Hashirama was extremely powerful, the descendant often has to deal with the expectations and fears that come with that power. Another theme is identity. Figuring out what it means to be related to Hashirama and how to live up to that legacy.
One common theme is the discovery of identity. The oc has to figure out who they are as a descendant of Gilgamesh.
A prevalent theme is honor. Lancelot was known for his honor, and the oc as his descendant is often faced with situations where they have to uphold or redefine what honor means in a different time or context. There may also be themes related to magic or the supernatural, especially if the fanfiction incorporates elements from the Arthurian legends that were associated with Lancelot, such as enchanted weapons or magical foes.
One possible theme could be identity. As the son of Leviathan, the OC would likely struggle with who he is and how he fits into the DxD world.
Often, themes of friendship and adventure might be common. Since they are fictional characters, they could go on various adventures together.
A common theme is the power and mystery associated with the female leviathan. Writers often explore how her presence affects the world around her, whether it's a fear - inducing effect on other characters or a sense of awe. Another theme could be her relationship with other beings, such as a human who tries to understand or befriend her.