I'm guessing that Nicholas is your interpretation of N's real name - or is it the actual canon name? (Personally, I think it's Noah, but I digress.)
There was one error that I saw, but it was rather insignificant compared to the oneshot as a whole.
"Bared" would imply that N showed his father his teeth (you know, how animals bare their fangs). "Bared it" makes even less sense. I think that you wanted to portray this as something more subtle (because he obviously doesn't want to show any signs of disrespect to his father), so "bore it" would be what you're looking for.
The noise annoyed Nicholas greatly, but he bared
it with gritted teeth.
I thought that it was a nice oneshot. Both N and his father were perfectly in-character, and that's what matters most when you put canon characters in your fic. The fic title fit perfectly as well. Aside from that one error, I couldn't find anything that seemed off.
Warmonger - A fantasy fic where peasants fight the wars of kings and lords, where monsters lurk in the night, and where depravity is not only expected, but encouraged...