That life threatening condition altered him and afterwards, it develops into a flame-like pattern and is his new Demon Slayer Mark. It remains that way and continues when he uses his abilities. This is a cool piece of mythos that will evolve as the story continues.
It appeared in the final fight as he was cutting off Gyutaro's head. But after the fight, it simply reversed back to it being his normal scar.
why tanjiro's head is so hard? Well, it's thanks to his genetics, as his mother, Ki Kamado, passed down the trait of a rock hard head to him. In fact, Nezuko heard a story about their mother. scaring away a boar by headbutting it, which shows that Ki was also very tough and fearless.
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.
Fighting in conjunction with Tengen, Tanjiro awakens his Mark while beheading the demon. It is said that only those who have survived life-threatening situations with a heart rate of 200 bpm, as well as a body temperature over 39 degrees Celsius, can awaken their Demon Slayer Marks.
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.
Demon Slayers who have awakened the Demon Slayer Mark are said to die upon reaching the age of 25. Michikatsu Tsugikuni speculated this to be because the mark exchanged the life span of its user for power.
Tanjiro certainly did a lot of good with his Water Breathing techniques, but his ability to use Sun Breathing both gives him an edge in battle and allows him to stand out among the incredibly powerful Hashira he fights alongside.
In the show, Tanjiro's father Tanjuro is shown with the exact same head "scar" as his son - and it seems unlikely that the explanation for this is that they both had the exact same scarring incident with a boiling hot kettle. As such, it's clear this is an inherited trait, and not the result of an unfortunate accident.
The black blade represents the Sun and is surrounded by prejudice and misconceptions, as historically every demon slayer who held it lived a short life. This could mean that Tanjirou is destined to break the pattern and set a new legend for black Nichirin blades in the Demon Slayer universe.
Thanks to Muzan, the demon form of Tanjiro has the fastest regeneration and the strength to avert all physical attacks. Moreover, just like his sister Nezuko, Tanjiro acquires resistance against the sun, making him completely invincible against any human.
During his life, Tanjuro was stricken by an unnamed disease that caused his body to be so skinny and frail, and his skin also appeared quite pale. In his later years, Tanjuro spent most of his days bedridden.
They fought demons and trained together, yet Inosuke never acquired a mark of his own. Inosuke did learn a great deal about his tragic past, but he didn't get as much of a chance to shine as his other friends when it came to displaying his talents.
Rengoku died before the Mark was introduced into the story. Similarly, Tengen was jacked up during the Entertainment District mission and retired immediately after. As he wasn't on the front lines anymore, he never had the chance.
During their battle, the former unleashed an attack that harmed multiple people in the entertainment district. Seeing the loss of human lives and the ceaseless destruction caused by the demon, Tanjiro's was overcome with rage, causing his eyes to start bleeding.
He had a disease that left his skin looking drab and made him weak. Nevertheless, Tanjuro was born with a birthmark on his forehead. Tanjuro wore a similar costume to his son, wearing a black checkered haori that was orange rather than dark turquoise, and wore Hanafuda earrings before passing them down to Tanjiro.
Tanjuro Kamado bore a mark on his head similar to Tanjiro's own scar, identifying Tanjuro as a chosen wielder of Sun-Breathing. This confirms Shinjuro Rengoku's implication that Sun-Breathing is a thing of destiny.
Tanjuro Kamado ( 竈門 かまど 炭 たん 十 じゅう 郎 ろう , Kamado Tanjūrō?) was the husband of Kie Kamado and the late father of Tanjiro, Nezuko, Takeo, Hanako, Shigeru, and Rokuta.
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.
The Kamado family has a connection to Sun Breathing, the original and the strongest breathing technique in Demon Slayer. While Nezuko cannot use this breathing style, her ancestry may have influenced her fire-themed Blood Demon Art and allowed her to stand out from other demons.
Gyomei and His Unusual Situation
Only two Slayers were ever known to be older than 25 with a Mark. Yoriichi lived till old age, likely due to him having been born with the mark and thereby avoiding the curse altogether. Gyomei on the other hand did not activate his Mark until he was already older than 25.
Among the finalists of the Final Selection, in his batch, Tanjiro Kamado alone unlocked his demon slayer mark. This implies that Tanjiro only has 9–10 years left before he kicks the bucket, seeing that he's more or less 16 years old by Chapter 204.
It is presently unknown why Yoriichi is immune to the Mark's effects, but it is presumably because he was the first one ever to have it and born with it naturally, unlike all other mark bearers who must train to attain it.