The following contains spoilers from Volumes 20 and 21 of the Demon Slayer: Kimetsu no Yaiba manga, available now in English via Viz Media.

Demon Slayer's main antagonist, Muzan Kibutsuji, has been alive for several centuries, which means he's experienced a lot. With his blood, Muzan can turn others into demons and seems like an unstoppable foe. However, the demon king seems to fear one thing: Tanjirou's hanafuda earrings.

A look into Muzan's past reveals that only one man ever came close to killing him. This demon slayer, while not a direct descendent of the Kamado family, is connected to Tanjiro in more ways than one.

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Muzan Kibutsuji's Past With the Demon Slayer Corps

Demon Slayer's Yoriichi Tsugikuni standing over defeated Muzan with sword in hand

In his brief flashback in Chapter 14 of the Demon Slayer manga, Muzan recalls a man with long hair and the same hanafuda earrings that Tanjiro wears. Then in Chapter 81, during the "Entertainment District" arc, Daki sees the same man through Muzan's memories because of his blood coursing through her body. The mysterious swordsman didn't make a complete appearance until Chapter 99 (or Volume 12) of the manga.

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. Yoriichi encountered Muzan and Tamayo in a bamboo forest, where the two engaged in combat. With Yoriichi's Sun Breathing technique, he beheaded Muzan and stopped his retentive abilities. However, he failed to completely kill the demon because Muzan promptly fled after being overpowered.

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Who Is Yoriichi Tsugikuni?

Yoriichi Tsugikuni holding his red Nichirin sword

Yoriichi Tsugikuni was known as the Demon Slayer Corps' strongest swordsman. He lived 500 years before the start of the anime, during the Sengoku era which was a golden age for demon activity. Yoriichi invented the first breathing style from which subsequent slayers derived their versions. His technique, known as Sun Breathing, later became the Hinokami Kagura Dance that the Kamado family practiced.

As a kid, Yoriichi was calm and reserved, hardly showing any emotions and keeping to himself. This aspect of him made Muzan fear the swordsman because Yoriichi never emitted any bloodlust or mal intent that most fighters did during battle. Instead, his calm nature made it difficult for his opponents to read him or predict his movements. Yoriichi is the younger twin of Michikatsu Tsugikuni, later known as Upper Moon 1 of the Twelve Kizuki: Kokushibo.

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How Is Yoriichi Connected to the Kamado Family?

Yoriichi looking forlorn on the left and determined on the right

Yoriichi was close friends with Sumiyoshi, an ancestor of Tanjiro. He saved the man from a demon attack, and the two remained close afterward. Yoriichi considered Sumiyoshi one of his closest friends, although his work as a demon slayer prohibited him from seeing the Kamado family often. After his banishment from the Demon Slayer Corps, Yoriichi confided his life's troubles to Sumiyoshi, who was in tears after hearing of his friend's struggles.

Sumiyoshi's daughter, Sumire, took a liking to Yoriichi and was very fond of the demon slayer. Her harmless affection to comfort him at one point inadvertently caused Yoriichi to break down crying. While practicing his Sun Breathing technique to entertain the little girl, Sumiyoshi watched and mesmerized all the movements. This technique later became the Hinokami Kagura, which passed through the family for generations.

Before Yoriichi left the Kamado family, he entrusted Sumiyoshi with his hanafuda earrings. 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.