Warhammer 40k- Primarch Overpowered

I actually don't know much I should know for the Warhammer 40k to write fanfiction for it but whatever. If I don't something or I get something wrong don't get mad. It's for the story. I am going to change the lore a lot. This is going probably a very overpowered primarch story. Overpowered humanity. Crossover technology.