Note: This Demon Slayer article contains spoilers.
Demon Slayer's Entertainment District Arc has arguably become the most admired arc of all time. It has slowly picked up the pace and unveiled various secrets As the story progresses, the restlessness of knowing more about Muzan Kibutsuji, the terrifying primary antagonist of the series, grows exponentially.
With the grand entrance of the despicable Muzan came some surprising specifics. Despite being the strongest demon alive and the first of his kind who has lived for thousands of years, some flaws come as obstacles in his journey to becoming the "perfect being" alive.
Why is Muzan afraid of Tanjiro in 'Demon Slayer'?
Tanjiro Kamado was just a nobody to Muzan until his encounter with him triggered his recollections about a person holding a Nichirin sword looking down on him. The sight of Hanafuda earrings on Tanjiro devastated him greatly, as they were the same earrings seen on the man who once defeated Muzan.
This mysterious man was none other than Yoriichi Tsugikuni, the first Demon Slayer and the inventor of the Sun Breathing technique. 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.
Hanafuda Earrings are a symbol of Sun Breathing users, traditionally passed down from generation to generation as a family heirloom. Since Yoriichi didn't have any successors, he gave it to his close acquaintance Sumiyoshi, the ancestor of the Kamado family.
From there, it subsequently got passed down throughout the generations in the Kamado family. At last, it got handed over to Tanjiro by his father to keep the family tradition alive.
The massacre of the Kamado family turned out to be intentional, as Urokodaki Sakonji confronts Tanjiro about Muzan, saying only the latter possesses powers to turn humans into demons. Eventually, Muzan made his first appearance in Episode 7 of Season 1, when he came across Tanjiro.
Subsequently, Muzan sends two underlings to annihilate the Demon Slayer wearing Hanafuda earrings.
In the long run, Muzan's obsession with perfection was obliterated by the inability to withstand the sun's exposure. The users of the Sun Breathing technique also posed a significant threat to him, as it is the only technique that exists to be lethal to him.
The Sun Breathing technique is also the founding legendary Breathing Style, out of which other Breathing Styles have been derived. It became Muzan's second major weakness.
For millennia, Muzan has been after Blue Spider Lilly. This mythical flower could grant him immunity against the sun so that he can get out in broad daylight without withering into ashes.
Akaza, the third upper-ranked demon moon, was sent by Muzan to find the whereabouts of the Blue Spider Lilly when he happened to run into the Flame Hashira, Kyojuro Rengoku, as shown in Episode 6 of Demon Slayer's Mugen Train Arc.