During his final struggle, and as a way to slow down the effects of the sunlight, Muzan adopts an enlarged demonic infant form, approximately as tall as a 3 story building when standing up. In this form he has very pronounced blood vessels across his body, small claws in his hands and fanged teeth in his mouth.
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.
For those who have watched only the first season of the anime, they will be surprised to know that Muzan turns into a woman in the second season. He keeps transforming in order to hide his true identity, and he's even known to change into an 11-year-old child to hide from Demon slayers.
Here Muzan is seen in his true, demonic form.
Demon Slayer: Kimetsu no Yaiba (TV Series 2019– ) - Haruka Shiraishi as Muzan's Daughter - IMDb.
Based on the familiarity in which the two conversed at the end of the story, and the fact that she was beside him when he fought Yoriichi, suggest that he did have some fondness for Tamayo, though why in never covered.
After he neutralizes Tamayo's poison and emerges from his flesh cocoon, Muzan's hair grows in length and turns white due to Shinobu's poison accelerating his age. He also grows numerous mouths all over his limbs, each one lined with a set of incredibly sharp fangs.
Muzan's Blood Demon Art is known as “Biokinesis.” It allows him to alter his body and shapeshift at will. Although all demons have this innate ability, Muzan elevates it by modifying his flesh and organs. Biokinesis is also the only reason Muzan has been able to hide from the Demon Slayer Corps for 1000 years.
2 His Body Is Weak To Certain Poisons
Poisons that did not exist before, like Tamayo's concoction of turning demons into humans, are the kind of poisons that Muzan's body isn't prepared for, as he's never been exposed to them before, and thus, his body has no idea as to how it should fight those poisons.
The only reason why Muzan hates Doma is because of his insincerity and lack of emotions. This might sound odd, expecting emotions from a demon, but even a demon has to retain such tendencies to keep things in order.
Muzan himself never thought highly of Daki and didn't reciprocate her devotion whatsoever, only ever talking to her to sweetly manipulate her into defeating Demon Slayers on his behalf.
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.
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.
Over his long lifetime, Muzan had a number of families. This includes the one he was born into before becoming a demon. Additionally, he took 5 wives, making each one's life miserable.
Akaza was also one of Muzan's favorite demons out of all his subordinates. The reason behind this is Akaza's extreme loyalty to his master. The Upper Rank Three demon was always sent away on special missions, something the Upper Rank 1 and 2 were often denied of.
Muzan's loyal wife and daughter seem like convenient test subjects to help him synthesize a cure for sunlight weakness as a backup in case the Blue Spider Lily is never found. The young girl might even be Muzan's biological child, a half-demon who gives off the scent of a human while having his essence in her.
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.
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.
Nezuko's Blood Demon Art is “Pyrokinesis,” which allows her to generate and manipulate special demonic flames created from her blood. It may be because of her unique nature, but her flames have a special ability, making them only harmful to demons and anything related to demons.
Nezuko and her older brother Tanjiro Kamado are the sole survivors of an incident through which they lost their entire family due to the Demon King, Muzan Kibutsuji, with Nezuko being transformed into a demon, but unexpectedly still showing signs of human emotion and thought.
Demon Body: Unique from other demons, he possesses seven hearts, and five brains.
It's true that in later episodes, Muzan also appears as a woman wearing a kimono when he's holding court over other Demons, most of whom are also wearing traditional Japanese clothing. This might have something to do with Demons (or “Oni”) themselves being part of traditional Japanese culture.
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. Susamaru accidentally shot Muzan's name aloud, attempting to defend his honor with her mind clouded.
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.