Blackfyre rightful owner

Hey guys I would like to start a role-playing campaign as Daemon II, the rightful ruler of the seven kingdoms, Targaryen. I see that bittersteel has Blackfyre and the black dragons crown. I see that I can steal the crown, but not the sword. How can I get a hold of it? I go to my diplomatic tree I don’t see the option of creating a artefact claim, I think it changes when you are not a ruler.