The fuming rage that Tanjiro felt from seeing Daki destroy human lives helped him push past his limits while also making him bleed through his eyes in anger, resulting in his eye colors changing to bloodshot red.
During his second encounter with Muzan Kibutsuji, he injures his right eye in the Infinity Castle. As the battle progresses, Tanjiro suddenly collapses and his wound begins to form a large mass over his right eye due to the poison inflicted on him by Muzan.
Thus, given how Mitsuri whispered into Tanjiro's ear, he immediately understood what had happened and was left overwhelmed at the thought of romantically getting close to the Love Hashira, which is what caused him to get a nosebleed.
Muzan refused to die and when he was finally about to breathe his last, he transferred all the blood to Tanjiro in the hope that he would become the successor and carry out his will.
Muzan injects Tanjiro with all of his blood, turning him into a demon.
The two known demons who broke free of Muzan's curse are Tamayo and Nezuko. Tamayo was able to break free of the condemnation when Yoriichi fought Muzan, nearly killing him in the process. During this weakened state, Yoriichi let Tamayo go, free her from Muzan, and free her from the curse.
Demon Body: Unique from other demons, he possesses seven hearts, and five brains.
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.
In the "Demon Slayer" anime series, demons cannot say the surname "Muzan" because it's forbidden in their society. Muzan Kibutsuji is the main antagonist of the Demon Slayer anime series, and he is the founder of the demon society and the source of the demon curse that plagues humanity.
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.
In the case of Tanjiro, the mark used to be just a graze, but during his fight with the Hand Demon during the Final Selection, the mark changed. That life threatening condition altered him and afterwards, it develops into a flame-like pattern and is his new Demon Slayer Mark.
To become a member of the Demon Slayer Corps, he had to clear the Final Selection. During this event, Tanjiro received a massive blow to his head from the Hand Demon that killed many of Urokodaki's students. This increased the size of his scar and changed the color as well.
Tanjiro — Imposter Syndrome & Self-Doubt
He blames himself for Rengoku's demise, and he wants to protect anyone from having to experience a fate similar to that of his own. At the start of Tanjiro's battle with Daki, we gain a lot of insight regarding his personal struggles.
In episode 6 of Demon Slayer season 2, Nezuko entered a berserk state to protect her brother Tanjiro, causing her body to develop into that of an adult. She loses her gag, grows a single horn and gains vine-like patterns across her body.
After Doma's demise, Kanao wears Shinobu's hair pin instead of Kanae's, as the latter was destroyed during the fighting. Furthermore she loses visibility in her right eye due to unleashing the final form of Flower Breathing, even though the eye is unaffected visually.
Though he eventually avenges his family during the final battle, he loses his left arm and right eye in the process while being fatally wounded. Both are restored during Muzan's attempt to turn him into his successor by transforming him into a demon, though with the aid of his friends, he becomes human once again.
Don't go! DON'T LEAVE ME HERE! Muzan speaks his final words as he loses control over Tanjiro.
His search came to an end when he discovered that Zuko was able to withstand the sun. While this is a great plot point, fans are confused as to how a demon whose main weakness is the sun can survive it. This is why Kamado Nezuko was able to stand in the sun despite being a demon.
As Nezuko can't speak Muzan's name out loud, she can't be affected by the curse.
Haruka Shiraishi: Muzan's Daughter.
However, in truth, as revealed after her death, Muzan sees Daki as a tool to keep her brother, Gyutaro, in line, deeming her to be the main reason why the "true" Upper Rank 6 was killed and in truth does not think highly of her and deeming her as someone not very intelligent.
Tsukito, Yorogi, Kuraku, and Isatsu all agree that they are loyal to Muzan.
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.