WARNING: The following contains spoilers from the Demon Slayer: Kimetsu no Yaiba manga, by Koyoharu Gotouge, available in English through VIZ Media.
As horrifying as he is mysterious, Muzan Kibutsuji is the ultimate evil in the world of Demon Slayer: Kimetsu no Yaiba. The original demon, and therefore the Demon King, Muzan's reign of terror in Japan has lasted for 1,000 years. A monster whose power makes even the deadliest demons quake with fear in his presence, Muzan is nearly immortal in the truest sense of the word. Able to survive decapitation and even the complete destruction of his body, along with the ability to create and control demons, Muzan is a nightmare beyond comprehension. And yet, despite being the Demon King and evil incarnate, Muzan, like all other demons, was once a human.
Roughly 1,000 years ago, during Japan's Heian Period, Muzan Kibutsuji was declared stillborn after his heart stopped beating multiple times inside his mother's womb. As he was about to be cremated, Muzan coughed and squirmed on the pyre, indicating this young, sick infant still had the will to live. However, Muzan would never fully escape his intimate relationship with death, as he was later diagnosed with a terminal illness that would take his life before he reached his 20th birthday. Refusing to surrender to death, Muzan sought out a physician who attempted to give the dying young man a few more years of life by experimenting on him.
After the physician treated Muzan with medicine made from a flower known as the Blue Spider Lily, Kibutsuji became enraged at the lack of results and murdered his doctor. As soon as Muzan spilled the doctor's blood, however, he realized he had gained incredible, inhuman strength. Additionally, Muzan instinctively realized that he would die if he were ever to be exposed to sunlight. Whatever medicine the physician had crafted using the Blue Spider Lily had effectively granted Muzan immortality, transforming him into the world's first demon.
Muzan quickly began to crave human flesh and had no issue devouring humans to assuage his hunger. However, while Kibutsuji openly accepted the gifts and costs of becoming immortal, he remained humiliated by his inability to survive exposure to sunlight. Muzan had spent his entire life on death's door, and now that imminent death no longer loomed over him, he found himself trapped by his new weakness, only able to live his life in the dark. Muzan was intoxicated by his immense power as a demon but refused to accept anything other than total immortality as the perfect being so that death would forever remain a stranger to him.
Having been granted immortality by the Blue Spider Lily, Muzan sought to replicate the doctor's work with the flower in order to find a way to eliminate his weakness to sunlight. Unfortunately for the Progenitor of Demons, the physician's notes were inconclusive, the information regarding the location of the flower and how to use it having died with the doctor. In the 1,000 years between his rebirth as a demon and the events of present-day Demon Slayer: Kimetsu no Yaiba, Muzan was never able to locate the Blue Spider Lily.
Muzan initially began creating more demons in hopes that one would be born with immunity to sunlight, but he began using them for other tasks as well. The Upper-Ranks of the Twelve Kizuki are the only demons he appears to trust or even almost like. Muzan clearly has a soft spot for Akaza by permitting his refusal to harm women and entrusting him to keep up the search for the Blue Spider Lily. He appears to have genuine trust in Kokushibo, Upper-Rank One, as they once traveled across Japan murdering users of Sun-Breathing -- the only Breathing Style that's capable of causing serious, permanent harm to Muzan's body.
While demon slayers and even demons themselves rightfully fear the Demon King, Kibutsuji's only fear is once again becoming the weak, dying creature he was in his youth. And even though he basks in the blood and suffering that he and his demons have wrought, he's not interested in conquering or destroying; he just wants to become the perfect being. He has committed countless acts of unspeakable evil during his long life, but his motivation is utterly human: he is afraid of death.
