In the anime
In the letter, he reveals to Tanjiro that the scar on his head is the mark of one who has been chosen to wield the core of all Breathing Styles — Sun Breathing.
Protagonist Tanjiro Kamado was the first demon slayer of his generation to acquire the Demon Slayer Mark. The Mark, which was once a burn mark that he acquired earlier in life, slowly changed over the course of the series to heavily resemble the Demon Slayer Mark that both Yoriichi and Tanjiro's father Tanjuro had.
Once awakened, the Mark can disappear after a period of time along with its effects, though it can be re-activated (as seen with Muichiro Tokito and Tanjiro Kamado) by meeting the same conditions or through training to maintain the 'Marked state' to prevent it from dissipating in the first place.
Tanjiro's Scar: Was it a birthmark or something else? Tanjiro's mark was not originally a mark, but a scar from rescuing his brother from a falling brazier. However, as he sets out to become the most potent Demon Slayer, his scar evolves into a mark.
In Demon Slayer, Tanjiro's scar has always been a topic of discussion among fans. Originally appearing as a burn mark, fans soon learn that it represents his powers as a true demon slayer. As the main protagonist of Demon Slayer, Tanjiro Kamado has always shown capabilities beyond an average human.
Though Tanjiro succeeds in making use of Water Breathing throughout the early parts of his adventure, he eventually discovers that he's better suited for Sun Breathing instead. Sun Breathing was the first Breathing Style utilized by Yoriichi Tsugikuni.
It is a rare blade and represents the sun; it also holds a theory that the demon slayers who own this sword are not to live a long life. Black swords are also supposed to hold all the colors, which means Tanjiro is to find his way through all colors and breathing techniques.
Towards the final arcs of the manga, Tanjiro is mortally wounded and dies during a fight with the Demon King Muzan. Though Muzan is also seriously injured, he uses the last of his powers to turn Tanjiro into a demon.
Yet, Rengoku's fight to the death, in which he was protecting his fellow demon slayers, did not result in him awakening a mark of his own. The argument could be made that only those with a sufficient skill level and latent power can produce a mark.
Shinobu Kocho is a capable Demon Slayer whose worthy of being the Insect Hashira. That being said, she's arguably the weakest Hashira when it comes to pure strength.
During the Sengoku Era, about 500 years before the current timeline of the series, Yoriichi Tsugikuni was well-known as the strongest demon slayer in existence. He is the progenitor of Breathing Styles, born with a demon slayer mark, which means he never had to go under special training to awaken it.
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.
Does Tanjiro lose his eye? Yes, Tanjiro loses his right eye. After being exposed to Muzan's poison, his eye is healed but he can't see out of it. Even after returning to his human form, the iris of his right eye remains a dull pink-gray color and the pupil black.
Tanjiro then noticed that the other civilians were either dead or badly injured. This enraged him, and he soon stopped breathing, pushing himself beyond his body's limits. His eyes subsequently started to fill with blood seeping into the sclera of his eyes.
As Muzan is about to shuffle off the mortal coil, he uses the last of his powers to turn Tanjiro into a demon. It was part of his nefarious plan all along! After all, it was theorized by the series big bads that Tanjiro's demonic potential surpasses even that of his sister, one of the strongest demons in existence.
Yet, for some reason, Tanjiro Kamado, the series' protagonist, never becomes a Hashira. Demon Slayer's protagonist Tanjiro Kamado starts out as an ordinary boy but has his entire life turned around after his family massacre.
Nezuko is the daughter of Tanjuro Kamado and Tanjiro's younger sister who is turned into a demon Despite Muzan Kibutsuji assumption that he has killed them all in his attempt to sire a sun-resistant demon, Nezuko survives as a demon with her mind mostly gone.
That being said, Tanjiro's rank is unknown, and his rank was last revealed in the Entertainment District arc. In that arc, they were at Kanoe, which is fourth from the bottom. Since Kiriya Ubuyashiki announced the disbanding of the organization, Tanjiro doesn't get the chance to become a Hashira.
Black blades are seen as a rarity, as demon slayers who wield them are doomed to die young. No wielder of a black sword has ever become a Hashira, either. However, this is all superstition.
The sword is primarily black, but it turns crimson red when the Breath of the Sun God (strongest breathing style) is used by both Tanjiro as well as Yoriichi.
Sun Breathing is considered to be the originator of all breathing techniques, which is perhaps why it's one of the most powerful breathing techniques around. This breathing style is part of the Kamado family's legacy and is taught from generation to generation, with the father teaching the move to their son.
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.
Moon Breathing is the demonic form of the primal Sun Breathing in Demon Slayer anime and manga. Developed by Kokushibo, aka Michikatsu Tsugikuni, this is the second most powerful breathing technique in the series that no human can use. Moreover, it also has more forms than any other type of breathing.