Muichiro Tokito
He eventually dies after succumbing to his wounds after battling Upper Moon 1, who is also his ancestor.
Mitsuri had already entered the battle with grave injuries, so Obanai forced her on the sidelines so she wouldn't get hurt anymore. Obanai and Tanjiro defeated Muzan. Unfortunately, the Serpent Hashira succumbed to his injuries. He and Mitsuri confessed their love and died together shortly after.
To sum things up, Tanjiro died fighting Muzan and was revived within seconds due to the transfer of blood from the Demon King to Tanjiro. He turns into a Demon and Kanao injects a certain medicine made from Wisteria flowers that turns him back to a human.
The Water Hashira, Giyu, survived the "Infinity Castle" arc after fighting beside Tanjiro against Akaza and Muzan.
Summary. The sun begins to rise and Muzan Kibutsuji is horrified at the sight of it. He unleashes another shockwave and blows Obanai Iguro and Sanemi Shinazugawa away, however, Tanjiro Kamado holds on but loses his left arm. Tanjiro loses his arm.
Tanjiro decapitates Akaza with the Hinokami Kagura: Setting Sun Transformation. Akaza was bewildered, wondering how Tanjiro did not have any fighting spirit, comparing this feeling to fighting against a plant. He acknowledges that Tanjiro surpassed his speed.
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.
Yoriichi Tsugikuni, the first swordsmen to ever use Breathing Styles, once nearly killed Muzan with Sun-Breathing, and Muzan has been both physically and psychologically scarred ever since.
Three Hashira Tanjiro never will beat in Demon Slayer
Even amongst his fellow Hashira, Giyu is unique in being the only one who develops a Breathing Style Form of his own. In this process alone, Giyu already sets himself apart from the other Hashira.
After defeating the Upper Moon 1, Gyomei quickly found himself fighting against Muzan. During the fight, his legs are crushed, and he eventually dies while creating an opening for Tanjiro to attack.
And finally, let's get to the number one saddest death in the franchise, and of course, that's the death of the brightly burning Flame-Breathing user that is Kyojuro Rengoku.
Giyu Tomioka and Sanemi Shinazugawa attend as the final surviving members of the Hashira, and together they they get a confirmation that not only are the demons all eradicated, but that the Demon Slayer Corps will be disbanding.
So far, the only Hashira we meet in the Demon Slayer series who has died is the Flame Hashira Kyōjurō Rengoku. Although he isn't the first of the elite squad to get to know Tanjiro, he is one of the first, and he accepts Nezuko and won't kill her despite the fact that she's a demon.
Mitsuri Kanroji, the Love Hashira, put up a good fight against the Demon King. She gave everything she had and even lost both her arms during the fight. However, she didn't stay alive for quite long. She succumbed to her injuries and died shortly after.
A millennium ago, during the Heian Era, Muzan was turned into a demon during an experimental treatment trying to cure his terminal illness, and his goal since then was to live without the fear of death and become truly eternal.
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.
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.
Tanjuro Kamado was the name of Tanjiro Kamado's father. He passed away before the event of the main story of the Demon Slayer. It is not mentioned how long Tanjuro had passed before the story, but in several flashback scenes, we can see him interacting with a younger Tanjiro.
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.
We first see Doma when he is still the Upper Moon Six while he is in the middle of eating a geisha. Later he becomes even stronger and rises to the position of Upper Moon Two. He dies because of the lethal amount of poison he absorbed through Shinobu Kocho's body followed by Kanao Tsuyuri beheading him.
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.
However, Doma remained unfazed by the deaths of his own parents. When he was twenty years old, Doma met Muzan Kibutsuji, who granted him power and turned him into a demon.