how did muzan become a demon

Muzan Kibutsuji became a demon after taking a desperate experimental treatment made from the rare Blue Spider Lily when he was a sickly human on the verge of death.

Quick Scoop: Muzan’s Demon Origin

• Muzan was a frail young noble during Japan’s Heian period, suffering from a mysterious, incurable illness that would kill him before 20.

• A physician gave him a last‑ditch medicine created from the legendary Blue Spider Lily to save his life.

• Believing the medicine had failed, Muzan flew into a rage and murdered the doctor before the treatment fully took effect.

• After the killing, he realized he had gained inhuman strength, regeneration, and immortality, but also an inability to walk in sunlight and a craving for human flesh—this is the moment he became the first demon.

• Because he killed the only doctor who knew the formula, Muzan then spent centuries hunting the Blue Spider Lily and creating more demons with his blood, hoping someone would evolve past the sun weakness for him to exploit.

In simple terms

He didn’t make a pact or get bitten; he became a demon by accident after a Blue Spider Lily treatment saved his life but twisted his body into something immortal, monstrous, and vulnerable to the sun.

Mini timeline

1. Human Muzan: chronically ill, terrified of dying young.

2. Experimental cure: takes Blue Spider Lily medicine from a doctor.

3. Outburst: sees no instant result, kills the doctor in anger.

4. Awakening: discovers superhuman abilities and demonic traits.

5. Aftermath: realizes he’s the first demon, then spends the next 1,000 years spreading his blood, creating demons, and searching for a way to conquer the sun.

From a story perspective, his origin turns a terrified, sick human into an immortal predator whose entire existence revolves around running from death and the sunlight that can still end him.

TL;DR: Muzan became a demon when a Blue Spider Lily–based medicine meant to cure his fatal illness instead granted him immortality, demonic powers, and a lethal weakness to sunlight, making him the first demon in the Demon Slayer world.

Information gathered from public forums or data available on the internet and portrayed here.