HosoVardenis
While coming back home, Tanjiro found that someone has killed his entire family and only his sister Nezuko was alive, the only clue being Two Scents that were there. When Nezuko tried to eat him and a Woman appeared, stopping that from happening. After that there was a fight between The Two, in which the Demon Slayer and by extension, Water Hashira, Giyu Tomioka decided to spare Life of Nezuko, seeing how she protected her brother and suggested Young Kamado to go and find Urokodaki, to be trained and become a Demon Slayer. After she left, suddenly a screen appeared in front of the Boy.
[Quest: "Siblings' Bond" Complete! You have met all the necessary Requirements! Welcome to The System, Tanjiro Kamado]
He followed the Demon Slayer's Instructions and found Urokodaki, where Tanjiro started his training.
During 2 years of training, Learning Water breathing, Tanjiro stumbled upon a strange tuning fork (Hibiki's Henshin Onsa), that he found. After passing The Final Selection, Getting his own Nichirin Sword and solving a Mystery of People's disappearances, he was given a completely new Quest and transported to different world.
Now, Tanjiro not only has to protects his world, but other different worlds as well. All the while, trying to find a cure for his sister and find True Culprit of his Family's Murder..