Muzan and Yoriichi

Permanent Trauma & Fear: Yoriichi is the only human to ever make Muzan feel true fear. Their encounter was so intense that 400 years later, Muzan still panicked upon seeing the Hanafuda earrings on Tanjiro Kamado, as they reminded him of Yoriichi. The "Monster" Perspective: Despite Yoriichi’s calm demeanor, Muzan perceived him as a monstrous, terrifying entity. Conversely, Yoriichi perceived Muzan’s aura as "magma erupting from a volcano". Failed Killing Blow: Yoriichi almost killed Muzan during their first fight, destroying most of his cells. Muzan only survived by splitting himself into 1,800 pieces, of which Yoriichi destroyed 1,500 instantly. Cellular Memory: Muzan's fear was so deep that even after the fight, his blood cells carried the memory of the encounter. This cellular memory was so strong that it caused Daki and Gyutaro (via Muzan’s blood) to fear Yoriichi centuries later.changed his tactics entirely, refusing to face Demon Slayers directly for centuries. He hid and ordered demons to kill anyone wearing Hanafuda earrings to prevent another "Yoriichi" from appearing. The "Selfless" Slayer: During the fight, Yoriichi activated the "Selfless State," meaning he showed no fighting spirit, anger, or intent to kill, which made it impossible for Muzan to detect his movements or prepare to defend himself. The Survivor's Guilt: Yoriichi viewed his failure to kill Muzan as his greatest mistake, apologizing to Tamayo for leaving the Demon King alive. This failure haunted him until his death, despite being in his 80s.