There is the Emperor and then the Primarchs. [And a bunch of ministratum ranks but those were filthy civilians and thus didn't matter. They could call themselves what they liked but the Adeptus Astartes did not have to honor them. But someone who had 1 v 1'd a heldrake? That was no petty civilian. ] My lord.

[Yeah sorry, Primarch Maedhros, sir, you're stuck now. The Codex is not clear on what to do with meeting foreign primarchs, but it is pretty clear in the necessity of rank and forms of address. ]

Or. [You know what he's going to say, just accept it at this point.] The forces of Chaos that control them are variable and changing. We could perhaps exploit that.