In this type of fanfiction, the main characters associated with the 11th Primarch can vary widely. There could be Inquisitors who are either investigating the 11th Primarch or working with him. These Inquisitors could bring their own motives and agendas. Tech - Priests might also be important characters as they could be interested in any new technology or knowledge that the 11th Primarch might possess. And of course, there could be Chaos Lords who see the 11th Primarch as a threat or an opportunity for their own dark purposes.
I'm not entirely sure as I haven't read it in detail. But generally, it might be about the journey, battles, and character development of Alpharius as a primarch.
There are fanfictions that focus on the Primarch Magnus the Red. For example, a story could be set during his time on Prospero, delving into his study of the warp and how his actions were misinterpreted by the Imperium. It could also show his complex personality and his love for knowledge.
One main theme is power. Combining the powerful abilities in Naruto with the might of the Primarchs often leads to exploring different levels and types of power. Another theme is identity. Characters may struggle with their new identities when influenced by Primarch - like qualities.
One interesting storyline could be the Necron Lord OC's struggle to regain their former glory in a universe that has changed much since their slumber. They might have to face new and powerful enemies while also trying to unite the disparate Necron factions under their rule.
In some fanfictions, Eldar characters might be paired with a Necron Lord OC. This creates an interesting dynamic as the Eldar are a very different and ancient race. They could be either enemies, with the Necron Lord OC seeing the Eldar as a threat to their plans, or they could form a tenuous alliance based on a shared goal, like stopping a Chaos incursion.
First, do a lot of research on the existing lore of the primarchs. Know their personalities, their backstories, and their relationships. For example, if you want to write about Vulkan, understand his connection to Nocturne and his forgiving nature.
One possible element could be the power - up. Just as primarchs are often extremely powerful, Percy might gain new powers or enhance his existing ones in this fanfiction. Another element could be the leadership aspect. Primarchs are leaders, and Percy might be put in a position where he has to lead in a more primarch - like way, perhaps leading a group of demigods in a more structured and strategic manner.
Well, it depends on how it's written. If it's a fanfiction, then it's someone's creative take on merging Percy Jackson with the idea of a primarch. Maybe the author saw some similarities between Percy's journey and the grandeur associated with primarchs. However, it could also be a really out - there concept that takes Percy in a completely different direction than his original story.