I'm pretty sure this a novel called path of ruin from spacebattle by an author called rictus.

I don't know why this story has so many five star ratings. Do people just stop reading this story after he fights the mountain? the MC goes from being someone who want to do everything in the shadows to fighting one of the most feared swordsman of that time and killing him. han wines and complains over and over again about not wanting to talk about it and why won't they leave him alone. And then here comes Dorne amd he gets even dumber and even more whiney about Prince Oberyn needing to leave him alone. Than he gets ridiculously standoffish with him. I had to stop reading because of how ridiculous it started to become. This story was so great until the author decided that his warrior-librarian needed to start acting how a child would.