Prologue: The Calm Before The Storm

(sorry if this Description is cringe) A young boy named Kamado Tanjiro is born with a fiery scar and a man living inside his head who Tanjiro would soon recognize as Master. When Tanjiro was just 5 years old this man told him of demons and their progenitor, Muzan Kibutsuji. He taught him how to kill them using a Nichirin Blade. He taught him Total Concentration Breathing: Constant and all about the different breath styles. (including Breath of the Sun oc and Breath of the Moon too, except for their offsets obvs) This man kept training and training Tanjiro until he was stronger than even himself. Tanjiro mastered all of the breath styles and even created a new one. But the question is will he be able to defeat his master's greatest enemy? And will he be able to survive the new threat that's looming, threatening to destroy everything in existence?