The demon Akaza from Demon Slayer was once beheaded and should have died, but to everyone's surprise, he survived. He is an Upper Rank Three of the Twelve Kizuki, which indicates that he is one of the most powerful demons, but this does not explain why he did not die.
As it has been well established, a demon’s neck is their weak point. However, simply slashing or decapitating a demon with any ordinary blade will not be effective. They can only be killed by the Curse of Kibutsuji if it was created by Muzan, sunlight, wisteria flowers, or the Nichirin Sword.
The latter is made from iron, sand, and ore gathered from the mountain closest to the sun. Scarlet iron and scarlet ore absorb sunlight and are thus effective against demons. Thus, Rui severed his own head and mocked Tanjiro, but was killed by Gyuu, who used his Nichirin blade.
But in Akaza's case, the Nichirin sword was unable to kill him, and he later died of his own will. How did this happen?
Disclaimer: This article contains spoilers from the Demon Slayer manga.
Akaza’s insane feat of surviving without a head in Demon Slayer, explained
Why was Akaza beheaded?
The plot of Demon Slayer revolves around Tanjiro, a young boy whose family was killed by a demon attack and who has since joined the Demon Slayer Corps with the goal of killing every demon. When Tanjiro and the Giyu entered the Infinity Castle to find the Demon King Muzan and Tamayo, they came across Akaza.
In a long fight sequence spanning several chapters of the manga, the Demon Slayer and the Water Hashira fought against the Upper Rank Three. Tanjiro also sought vengeance for the death of Rengoku at the hands of the demon.
The fight reached a point where Tanjiro gained a lot of strength and speed after seeing the Transparent World, allowing him to enter the Selfless State. He became invisible to Akaza's Compass Needle, allowing him to decapitate him with the Hinokami Kagura: Setting Sun Transformation.
How did Akaza survive the beheading?
In Demon Slayer, Akaza displayed a magnificent feat of perseverance immediately after having his head severed. He grabbed his head and attempted to reattach it because he wanted to continue fighting, grow stronger, and reach the Selfless State.
Giyu immediately tried to stop it by piercing the head with his broken Nichirin sword, but it was in vain. Even as the head fell to the ground and disintegrated, Akaza's willpower kept his body alive. As the fight raged on, he sutured the wound and a new head began to sprout.
It was revealed that Akaza's traumatic memories of failing people, particularly his father when he was a human, gave him the willpower to keep striving even after being decapitated as a demon. There were also memories of Keizo and Koyuki that unsettled him, eventually convincing him to stop fighting and accept death. Otherwise, Giyu and Tanjiro might not have survived.