Verbs in -nu

So, I've been watching Vinland Saga, and while I can understand anything more or less, there's something I really don't get. I've seen many times the verb (other than 死) end with ぬ, particularly when I saw people talking with the king. Is it a formal/archaic way? What does it mean? Here's an example: His Highness is in an unfamiliar battlefield and is under a lot of stress…

I also remember I saw 落ぬ or something like that. Any help?

by Kinulidd0