Yoriichi had lost his wife and unborn child to demons, and he had a fateful encounter with Muzan Kibutsuji, the Progenitor of Demons. Yoriichi defeated Muzan in an instant, but he had a question for him before eliminating him, which allowed Muzan to escape.
Muzan's experiment on Nezuko worked out exactly as he'd hoped, but unfortunately for him, he picked the wrong family to mess with, as Tanjiro joining the Demon Slayer Corps is what ultimately led to Muzan's defeat and death.
Moreover, Tanjiro's appearance somewhat resembled Tsugikuni. 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.
There is much speculation about the relationship between Tanjiro's father and Muzan. Some believe that Muzan was Tanjiro's father, while others think they may have been brothers. However, there is no concrete evidence to support either theory.
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.
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.
Tanjiro's connection to Yoriichi is not through blood relation, but through their similar appearances, which is due to the divine nature of Sun Breathing. (Also Read: Did Demon Slayer hire a Bollywood director for Season 3? Fans speculate!)
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.
Without the image of his father's dance burned deep inside his memory, Tanjiro won't be able to use the technique that will be crucial in the effort of defeating Muzan Kibutsuji. Other than the Hinokami Kagura, other techniques that Tanjiro learned from his father are the Transparent World and the Selfless State.
So, not only could Tanjuro use his mastery of the Hinokami Kagura for his dance, but he was also an excellent warrior despite his poor health. Tanjuro Kamado bore a mark on his head similar to Tanjiro's own scar, identifying Tanjuro as a chosen wielder of Sun-Breathing.
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.
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. However he is able to instantly regrow his head.
Tanjiro's ancestor was a good friend of Yoriichi and arguably was one of his only true friends. Therefore Yoriichi gave the earrings as a gift, and since then they have been passed down the generations until they reached Tanjiro.
The only reason why Muzan hates Doma is because of his insincerity and lack of emotions. This might sound odd, expecting emotions from a demon, but even a demon has to retain such tendencies to keep things in order.
Muzan spared Nezuko most likely because she expects something from turning a member of the Kamado family. True enough, Nezuko grew and transformed over the series. She demonstrated her Blood Demon Art during the fight with Rui. Nezuko then did her Demon Transformation in the fight against Daki.
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.
He was a man who received no formal training in combat and managed to take down a bear with a short axe. To sum things up, Tanjuro was one of the strongest characters aside from Yoriichi, given that he was able to access the Selfless State for hours on end.
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.
Tanjuro Kamado a recurring flashback hero from Demon Slayer : Kimetsu No Yaiba. He was the husband of Kie Kamado, and the father of Tanjiro, Nezuko, Takeo, Hanako, Shigeru, and Rokuta. Tanjuro was a flame dancer who used to dance the Hinokami Kagura during the winter.
1 Muzan Regretted Killing His Doctor
He shows his most human side to fans, wherein he admits to regretting murdering his doctor. He comments that had he been patient, the doctor would have cured him, and he would never have turned into a demon.
The sword was once called The Unfading Light of the Lake, and it only turned from silver-and-gold to black after being filled with Berserker's madness and resentment over not being able to protect King Arthur and Guinevere, since Berserker was once the Sir Lancelot.
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. The Upper Rank Three demon was always sent away on special missions, something the Upper Rank 1 and 2 were often denied of.
Demon Slayer's season 3's latest episode has left fans on edge with Tanjiro's discovery of Yoriichi Tsugikuni's Type Zero sword.
Tanjiro and his father look extremely similar to each other. Fans think that Tanjiro was related to the legendary swordsman since he had the same earrings, and their appearances are similar as well. However, Tanjiro is not a descendant of Yoriichi Tsukiguni.
As it turned out, the scar on Tanjiro's forehead did have some significance to it, rather than simply being a Todoroki-esque burn from scalding water. Tanjiro wasn't born with this scar on his forehead. Rather, he acquired it when tackling his younger brother out of the way of a falling pot of boiling tea.