The Demon King Muzan was the first ever demon to have come into existence. He's the reason other demons exist as he was able to turn humans into demons. This demon has been living for over 1000 years in an attempt to find Blue Spider Lily, which would grant him immunity from the sun.
Muzan Kibutsuji was a distant relative of the Ubuyashiki family and became the first demon by accident. A generous doctor from the Heian Period cursed him with demonic powers, transforming him into a devil. Muzan then injected Tanjiro with his blood, turning him into a demon as well.
1) Kokushibo
Kokushibo stands at Upper Rank 1 in the Twelve Kibuki. He is the only demon to have a breathing style in the Demon Slayer series. In fact, Muzan views him as more of a business partner and one of his closest allies.
Tanjirou and Giyuu are the first one to approach him. There they fight Muzan Together without stopping. They realized that the only way to kill Muzan is to drag him into the sun.
Using this opportunity, Gyomei Himejima appears and destroys Muzan's head. However he is able to instantly regrow his head.
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.
How Demon Slayer's Muzan Originally Became The First Demon - IMDb. The big villain of Demon Slayer, Muzan Kibutsuji, isn't just the mastermind behind a lot of the plots that the heroes try to stop; he's also literally the origin of nearly every demon they encounter, as a result of his ability to create more demons.
6) Daki - 113 years
Luckily Uzui, along with Tanjiro, Inosuke and Zenitsu intervened and ended up killing the siblings. She's quite child-like and throws tantrums when her brother is around. She had been living as a demon for about 113 years until Zenitsu and Inosuke decapitated her.
After the deaths of Daki and Gyutaro, Kaigaku was promoted to become Upper Moon Six of the Twelve Demon Moons due to his power and confronted Zenitsu as a rematch he always desired.
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 Muzan favored Gyutaro so much, he chose to incorporate the latter's sister Daki as well. The demon king believed that she was the only tool that could be used to control Gyutaro. However, he deemed Daki weak and believed she was the prime reason for her brother's death.
Akaza is an upper-rank-three Demon and one of the most faithful minions of the Demon king Muzan Kibutsuji.
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.
In a nutshell, Muzan, being the progenitor of demons, is not scared of Doma. However, he is irritated by his inability to decipher Doma's thoughts and feelings, which is the ultimate control he has over all of his creations. Upper-Rank Two: Doma is a nihilistic maniac and a misanthropic cold-blooded demon.
After various battles within the Dimensional Infinity Fortress the remaining Upper Moons are killed by the hands of various Demon Slayers. The last member, Upper Moon Four Nakime, ends up being killed by Muzan Kibutsuji himself to stop the Dimensional Infinity Fortress from being controlled by his enemies.
Gyokko, the Upper Rank 5 demon in Demon Slayer, wows fans with his eerie look and abilities. As the 5th Upper Rank in Twelve Kizuki, he's a formidable opponent. Gyokko, the Upper Rank 5 demon in the beloved anime series Demon Slayer, has captivated viewers with his eerie appearance and powerful abilities.
Is upper rank one Tanjiro's father? No, they are not related. Tanjiro, by PURE COINCIDENCE, heavily resembles Yoriichi Tsugikuni, the creator of Breath swordsmanship, the only other person to use Sun Breathing, and the only person Muzan ever feared.
Thus, Muzan created demons for hundreds of years even when he didn't want to turn someone. 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.
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.
1 Muzan (Demon Slayer)
Without that antidote, it would have been impossible to kill him. So despite all her strength and humanity, Nezuko would have easily lost to Muzan.
Muzan can detect the locations of his demons and can also read their thoughts. However, Nezuko and Tamayo are the only exceptions in the entirety of Demon Slayer, over whom Muzan has no control.
Kokushibo was the final Upper-Rank to die during Demon Slayer's "Infinity Castle" arc. Muichiro, Genya, Sanemi Shinazugawa and Gyomei Himejima fought together against Kokushibo, though Muichiro and Genya were no match for him and were killed.