These earrings became symbolic of Yoriichi's legacy, and Sumiyoshi ensured they would be remembered through his family. The earrings had such an impact that 500 years later, Muzan still feared them and the original swordsman who wore them.
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'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.
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.
Even 400 years later, Muzan's fear of Yoriichi remained fully intact, as simply laying eyes on Tanjiro's hanafuda earrings caused him to recall their clash.
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.
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.
Curse of Kibutsuji
This curse besotted any who partake of Muzan's blood must be careful. 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.
Muzan's ultimate goal is to attain an immortal body. He wanted to be able to walk in the sunlight and live forever. In order to gain this, he needed two things: the Blue Spider Lily and the body of a demon who could withstand the sun.
Tanjiro certainly did a lot of good with his Water Breathing techniques, but his ability to use Sun Breathing both gives him an edge in battle and allows him to stand out among the incredibly powerful Hashira he fights alongside.
It was after Daki unleashed an attack and harmed multiple people, Tanjiro was extremely angry at this that his eyes were turning blood-shot red.
In the Demon Slayer manga, Sumiyoshi would intently watch Yoriichi perform all the forms of the Sun Breathing technique. Just before Yoriichi left, he gave his Hanafuda earrings to Sumiyoshi who promised to keep them and pass them down to every generation.
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.
However, Tanjiro is not a descendant of Yoriichi Tsukiguni. Tanjiro is the descendant of someone who knew Yoriichi personally and was a good friend of his as well. Sumiyoshi was rescued by Yoriichi when a demon threatened his life. Since then Yoriichi often met him and they became close acquaintances.
Yoriichi was incredibly proficient in his Sun Breathing, being able to take down demons effortlessly and even overwhelm the Demon King and strongest Upper Rank.
For those who have watched only the first season of the anime, they will be surprised to know that Muzan turns into a woman in the second season. He keeps transforming in order to hide his true identity, and he's even known to change into an 11-year-old child to hide from Demon slayers.
However, she wasn't immune to the sunlight initially, as she got burned badly at first. So, in conclusion, her coming from a Sun Breathing user's lineage and not relying on human substance are perhaps the reasons for her immunity.
The Blue Spider Lily medicine was also the cause behind Muzan's demonification, granting him a strong body and possibly changing his blood to have transformative properties, enabling Muzan to create more of his kind as well as hold dominion over them.
What is the disease of Muzan? Muzan was born in Japan's Heian era, to a constant struggle to survive on account of his frailty. His heart stopped multiple times in his mother's womb, and when he first entered the world, he was deemed stillborn, having no pulse and not breathing.
Don't go! DON'T LEAVE ME HERE! Muzan speaks his final words as he loses control over Tanjiro.
He is the big bad guy (first demon) and in the manga and cosplay world, he is one of the most popular character. Also is full name is “Muzan Kibutsuji” and contains the kanji for “demon dance” (鬼き舞ぶ辻つじ kibutsuji?) and “miserable, pitiful, merciless”(無む惨ざん muzan?).
Fun fact: The reason why Enmu was able to say Muzan's name out loud without dying is because they're married!
@IsekaiCitizen Yoriichi and the kamado family are not related but the most common and logical reason for Muzan killing Tanjiro's family is revenge.
As he gives all of his blood to Tanjiro, Muzan says that if the boy survived this he would become the strongest Demon ever. Since he was the brother of Nezuko Kamado who was the first Demon to develop an immunity to sunlight and also the only one capable of using the Breath Style of Yoriichi Tsugikuni.
Nezuko is at least stronger than Daki, meaning that if Muzan could bring her back under his control, she would certainly qualify for the most prestigious title a demon can hold: a member of the Twelve Kizuki. Thanks to Nezuko's remarkable strength, she would have to be an Upper-Moon.