Muzan Kibutsuji has done some terrible things in Demon Slayer, but his slaughter of Tanjiro's family is one of the most mysterious.
Tanjuro passed away from an illness sometime before the events of Chapter/Episode 1 of Demon Slayer. Tanjiro remembers his father as a gentle, frail man who would dance the Hinokami Kagura every year, even as his health began to decline.
However, in the Demon Slayer manga Muzan reveals his motivation for attacking the Kamado family. It turns out that Muzan attacked the Kamado family and specifically turned Nezuko into a demon in an attempt to create a demon that would be able to withstand being in the sunlight.
Thanks to Somiyoshi, and later on Tanjuro who passed it down to Tanjiro, the world is left with a weapon to defeat Muzan Kibutsuji, long after Yoriichi passed away. Those are ultimately the role of Tanjiro's father, Tanjuro Kamado, in the story of the Demon Slayer.
While Demon Slayer Chapter 1 doesn't provide an answer to who killed Tanjiro's family, this is answered a few chapters later. Muzan Kibutsuji, the progenitor of all demons and indirectly Demon Slayers, is revealed as the one who turned Tanjiro's family.
So the only reason Muzan is afraid of Tanjiro is because of the Hanafuda earrings he possesses, which acted as a reminder of his downfall. nxt. Hanafuda Earrings are a symbol of Sun Breathing users, traditionally passed down from generation to generation as a family heirloom.
Tanjiro is considered to be Muzan's main nemesis throughout the series. After murdering the Kamado family, he was personally sought out by Tanjiro in Asakusa, Tokyo when the Demon Slayer recognized his scent from the scene of the murder.
The demon slayers successfully defeated Muzan.
Before he is able to regenerate fully, Tamayo appears and uses her Blood Demon Art to keep him in place, while using a drug that can turn him into human. Using this opportunity, Gyomei Himejima appears and destroys Muzan's head.
Since he didn't know anything about the Blue Spider Lily medicine, he needed others to find the plant for him by happenstance. This is the general reason why Muzan turned Nezuko into a demon. More specifically though, was the fact that Nezuko was of Kamado blood.
Unfortunately, Gyutaro spots Inosuke and stabs him right through where his heart should be with a sickle imbued with deadly poison.
Muzan refused to die and when he was finally about to breathe his last, he transferred all the blood to Tanjiro in the hope that he would become the successor and carry out his will.
Muzan's Descent Into Evil in Demon Slayer
Whatever medicine the physician had crafted using the Blue Spider Lily had effectively granted Muzan immortality, transforming him into the world's first demon. Muzan quickly began to crave human flesh and had no issue devouring humans to assuage his hunger.
To sum things up, Tanjiro died fighting Muzan and was revived within seconds due to the transfer of blood from the Demon King to Tanjiro.
We know that Tanjiro was able to completely outclass Akaza for a few minutes due to his access to the Selfless State and the Transparent World. It was in this flashback when fans realized that Tanjiro was able to learn this because of his father.
no. Yoriichi is not Tanjiro's dad, that would be Tanjuro. And Yoriichi is not related to the Kamado clan at all.
The curse prevents Muzan's name from being spoken aloud, and any who do will meet a horrible demise. One demon, Susamaru, faced this reality when battling Tamayo. Susamaru accidentally shot Muzan's name aloud, attempting to defend his honor with her mind clouded.
During his final struggle, and as a way to slow down the effects of the sunlight, Muzan adopts an enlarged demonic infant form, approximately as tall as a 3 story building when standing up. In this form he has very pronounced blood vessels across his body, small claws in his hands and fanged teeth in his mouth.
Don't go! DON'T LEAVE ME HERE! Muzan speaks his final words as he loses control over Tanjiro.
1. Gyomei Himejima. Gyomei Himejima, the Stone Hashira, is the strongest Hashira in the Demon Slayer corps in terms of strength and swordsmanship. He is the only person who uses spiked flails and an axe instead of standard swords to kill demons.
1 Muzan (Demon Slayer)
Without that antidote, it would have been impossible to kill him. So despite all her strength and humanity, Nezuko would have easily lost to Muzan.
Akaza was also one of Muzan's favorite demons out of all his subordinates. The reason behind this is Akaza's extreme loyalty to his master.
Apart from that, Muzan doesn't hate Akaza because of his obedience. The only Demon he despises the most is the Upper Rank Two: Doma. It is quite a shocker that the second most potent Demon in the Demon Slayer is not Muzan's favorite.
Some of the characters who have faced Yoriichi and survived
This happened due to a treatment that had gone wrong. While he was being treated, Muzan killed the doctor because he was not able to observe any results. This turned him into one of the strongest beings that could defeat multiple Hashiras in one go.