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.
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.
It is revealed that Muzan's true fear of dying and immense desire to live and survive was due to him being born "dead," where his heart would constantly stop in his mother's womb and was assumed stillborn until he fought to breathe before his cremation.
In Muzan's flashback, we see a shadow resembling a grown-up Tanjirou with a similar scar and Hanafuda earrings. This led many of us to believe that it was Tanjiro's father who had defeated Muzan back then and was secretly the strongest Demon Slayer to ever exist.
Muzan Kibutsuji is so afraid of Tanjirō Kamado because Tanjirō is the successor of the Hinokami Kagura dance, which is actually the Sun Breathing Style used by Yoriichi Tsugikuni to defeat Muzan centuries ago.
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.
Since Muzan favored Gyutaro so much, he chose to incorporate the latter's sister Daki as well. The demon king believed that she was the only tool that could be used to control Gyutaro. However, he deemed Daki weak and believed she was the prime reason for her brother's death.
Muzan Kibutsuji has done some terrible things in Demon Slayer, but his slaughter of Tanjiro's family is one of the most mysterious.
Showing a horrible facial burn, Muzan reveals he injected his blood into Tanjiro on his earlier attack, and that his body will be destroyed, boasting that he is already dead.
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.
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.
The only thing he's weak to is the sun. That is why he turned other humans into demons and spread his blood so that one of them might find a solution against the sunlight, so he can become truly perfect.
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.
The demon slayers successfully defeated Muzan.
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.
The episode begins with Tanjiro encountering the ghost of Yoriichi Tsugikuni, the creator of the Sun Breathing technique. Through a series of flashbacks, we learn about Yoriichi's tragic past and how he met the Kamado family.
Muzan Kibutsuji may have tried to kill Nezuko by exploding her after injecting a lot of his blood into her veins. But she was somehow able to survive with that much of Muzan's blood, that's what possibly made her a powerful demon.
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.
Towards the end of the Demon Slayer manga, it took the combined efforts of multiple Hashiras including Tanjiro, Inosuke, Zenitsu and the poison made by Tamayo to be able to defeat Muzan. Even then, the group barely managed to kill him and a lot of lives were sacrificed in the process.
if i'm not mistaken, the reason why muzan shape-shift is to hide his real identity or his face from the demon slayers. I think this is also the reason why the pillars were shocked when oyakata-sama revealed that tanjiro had a brief encounter with muzan.
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.
In the series, he died quite early due to an illness, and this led to Tanjiro taking over his responsibilities quite early in his life. Despite looking quite frail and weak, he was able to perform the dance for the fire God for hours on end without feeling tired.
Haruka Shiraishi: Muzan's Daughter.
Tsukito, Yorogi, Kuraku, and Isatsu all agree that they are loyal to Muzan. and show the utmost respect to him.
The warrior Muzan feared was a swordsman named Yoriichi Tsugikuni. Yoriichi was the only slayer before Tanjiro who came close to killing Muzan, and the flashback of their fight occurred in Chapter 187.