I read this novel quite a long time ago and really wanted to re read it. I remember that the mc’s parents died and he stayed at their grave cleaning it every day for 5 years( it may have been 10, I can remember it too well ), everybody thought he became dumb due to their death and pitied him, but in actuality he was fulfilling the promise he made to his dad, to practice this breathing Technique and not to retaliate for however many years. Anyway after that many years he joins the trials or tests to join the school/martial arts academy and surprises everyone with his prowess. I think, he went and did something bad? And ended up in confinement where he spoke to his soon to be teacher(he didn’t know that yet). The teacher had blue hair if I remember correctly. I think later on he becomes a ‘sword envoy’ when he joins the war agains demons or maybe another empire, I can’t remember. that’s all I really remember, thanks.