Demon Curse: Muzan places a curse on all demons that have assimilated his blood. If any demon divulges his last name "Kibutsuji" out loud, the curse activates, which causes their bodies to stop regenerating and allowing Muzan's cells inside the demon to take over and kill them, regardless of their original intent.
Curse of Kibutsuji
This curse besotted any who partake of Muzan's blood must be careful. The curse prevents Muzan's name from being spoken aloud, and any who do will meet a horrible demise. One demon, Susamaru, faced this reality when battling Tamayo.
What is the disease of Muzan? Muzan was born in Japan's Heian era, to a constant struggle to survive on account of his frailty. His heart stopped multiple times in his mother's womb, and when he first entered the world, he was deemed stillborn, having no pulse and not breathing.
The only thing he's weak to is the sun. That is why he turned other humans into demons and spread his blood so that one of them might find a solution against the sunlight, so he can become truly perfect.
The first demon that is said to have existed was Muzan Kibutsuji. The one who turned him into a demon was a generous doctor from the Heian Period, who wanted to save Muzan from death since, at the time, he was diagnosed with a disease which would kill him before he turned twenty.
Even 400 years later, Muzan's fear of Yoriichi remained fully intact, as simply laying eyes on Tanjiro's hanafuda earrings caused him to recall their clash.
Since he didn't know anything about the Blue Spider Lily medicine, he needed others to find the plant for him by happenstance. This is the general reason why Muzan turned Nezuko into a demon. More specifically though, was the fact that Nezuko was of Kamado blood.
It is revealed that Muzan's true fear of dying and immense desire to live and survive was due to him being born "dead," where his heart would constantly stop in his mother's womb and was assumed stillborn until he fought to breathe before his cremation.
Events. It is revealed that Tamayo's drug had an aging potion mixed in along with the Demon antidote. Due to this, Muzan has aged 9,000 years within the span of three hours.
Being a demon, Muzan is capable of living for many, many years. However, unlike other demons that have aged over hundreds of years. Muzan is well over 1000 years old. Despite achieving an age that no one else has, his appearance makes him look like an adult man in his early to mid-30s.
The explosion, which was enhanced by spike traps, manages to injure Muzan. Before he is able to regenerate fully, Tamayo appears and uses her Blood Demon Art to keep him in place, while using a drug that can turn him into human. Using this opportunity, Gyomei Himejima appears and destroys Muzan's head.
Technically, the elder of the two Sakuras` was actually Muzan`s child, by way of his mistress (Lady Jade Wong), who had one of her eggs inserted into Anne`s body.
Abilities. Demon Body: Unique from other demons, he possesses seven hearts, and five brains. These are responsible for his powerful vitality, should any of them be harmed, he will be severely weakened.
Fun fact: The reason why Enmu was able to say Muzan's name out loud without dying is because they're married!
History. Doma was born with beautiful and rare rainbow colored eyes. Because of this unusual trait, his parents thought that he was able to hear the voices of God and was groomed to be the religious leader of the Eternal Paradise Cult, a position that he held until his death.
However, she wasn't immune to the sunlight initially, as she got burned badly at first. So, in conclusion, her coming from a Sun Breathing user's lineage and not relying on human substance are perhaps the reasons for her immunity.
Sunlight: Muzan is noted to be immune to being killed via decapitation, as seen when Gyōmei destroyed his head and he still managed to regrow it. Thus the only other known way that he can be destroyed is by sunlight.
Simple. He killed the real father and replaced him.
Hoshiko Kibutsuji. The first female demon [of a different orgin], and the younger sister to Muzan Kibutsuji. She loves her brother and was the one who had attacked that man who wore the hanafuda earrings.
Yoriichi was incredibly proficient in his Sun Breathing, being able to take down demons effortlessly and even overwhelm the Demon King and strongest Upper Rank.
@IsekaiCitizen Yoriichi and the kamado family are not related but the most common and logical reason for Muzan killing Tanjiro's family is revenge.
As he gives all of his blood to Tanjiro, Muzan says that if the boy survived this he would become the strongest Demon ever. Since he was the brother of Nezuko Kamado who was the first Demon to develop an immunity to sunlight and also the only one capable of using the Breath Style of Yoriichi Tsugikuni.
Muzan's ultimate goal is to attain an immortal body. He wanted to be able to walk in the sunlight and live forever. In order to gain this, he needed two things: the Blue Spider Lily and the body of a demon who could withstand the sun.