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.
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.
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.
Originally a human, Muzan suffered from a terminal illness, during an experimental treatment, Muzan became the first ever demon. He is also the progenitor of most other Demons and the leader of the Twelve Kizuki. He is also the archenemy of the Kamado family and both Tanjiro Kamado and Nezuko Kamado.
Demon Slayer Mark: Yoriichi is the progenitor of the Demon Slayer Mark, being born with it, possessing the tremendous physical abilities granted by its effects all throughout his lifespan. Unlike all other Marks users in his era, Yoriichi was able to defy fate and lived to about 85 years of age.
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.
Kokushibo is reserved, silent, and aloof, maintaining an aura of unnerving tranquility and mystery that compliments his position as Upper Rank One.
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.
Yoriichi Tsugikuni was the first Demon Slayer and also the one who invented Breathing Techniques in the first place. Yoriichi was born with the Demon Slayer mark and prowess in swordsmanship. His powers were beyond comparison as he defeated Muzan once and had him on the verge of death until he managed to escape.
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.
During his final struggle, and as a way to slow down the effects of the sun, Muzan adopts an enlarged demonic baby 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.
Tanjiro Kamado - Wikipedia
. Muzan speaks his last thoughts and turns Tanjiro into the new demon king and subsequently his spiritual successor.
Fun fact: The reason why Enmu was able to say Muzan's name out loud without dying is because they're married!
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.
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.
Over a century ago, Doma held the position of Upper Rank Six ( 上 じょう 弦 げん の 陸 ろく , Jōgen no Roku?), and during that tenure, he turned Gyutaro and Daki into demons and introduced them into the Twelve Kizuki.
However, Tanjiro is not a descendant of Yoriichi Tsukiguni. Tanjiro is the descendant of someone who knew Yoriichi personally and was a good friend of his as well. Sumiyoshi was rescued by Yoriichi when a demon threatened his life. Since then Yoriichi often met him and they became close acquaintances.
Kokushibo (黑死牟) is the secondary antagonist of the manga/anime series Demon Slayer: Kimetsu no Yaiba. He is Upper Rank One of the Twelve Kizuki, and the archenemy of his younger twin brother, Yoriichi Tsugikuni.
Yoriichi combined all his forms into one strike and turned his blade bright red, enabling him to cut through all of Muzan's weaknesses and overpower him.
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.
ENMU was accredited by the North Central Association of Colleges and Secondary Schools as a four-year liberal arts college in 1947. Graduate work leading to the master's degree was added in 1949, and the institution was officially designated Eastern New Mexico University in 1955.
That being said, Tanjiro's rank is unknown, and his rank was last revealed in the Entertainment District arc. In that arc, they were at Kanoe, which is fourth from the bottom. Since Kiriya Ubuyashiki announced the disbanding of the organization, Tanjiro doesn't get the chance to become a Hashira.
Sakonji Urokodaki is one of the playable characters in Demon Slayer: The Hinokami Chronicles. A former member of the Demon Slayer Corps, Urokodaki is Tanjiro's master and he was also the master of Sabito and Makomo before they died.