Supreme Son-in-Law Emperor

Xiao Nan, humiliated and despised as the live-in son-in-law of the wealthy Wang family, endures years of ridicule from his cold wife and scornful mother-in-law.  Bound by a mysterious oath to his late master, Xiao Nan conceals his true abilities, silently bearing the torment.    
But on the night he performs a solemn ritual to honor his master's memory, assassins close in, aiming to annihilate the Wang family.  As the moonlight illuminates his path, the "worthless son-in-law" takes steady steps forward.    
What secrets has he been hiding all these years, and will this night mark the beginning of his revenge?