Hey ~ Seeing your needs, I can recommend the novel, Demon Hunting Bureau, to you. Although it is an Oriental fantasy novel, it also meets the requirements of the plot you described. After the protagonist's parents died in a car accident, the story related to demons began. Moreover, after the protagonist received the Dragon King's inheritance, his strength increased greatly. He can kill anyone who stands in his way. It completely fits the plot that you described as "his brain may die at any time." I hope you like this fairy's recommendation. Muah ~😗