It was after Daki unleashed an attack and harmed multiple people, Tanjiro was extremely angry at this that his eyes were turning blood-shot red.
During Tanjiro's fight against Daki, he was seething with rage. He managed to build up so much anger that his eyes started to bleed profusely. When he realized that the nearby civilians were becoming collateral damage, he was enraged.
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.
Of course, Rengoku offers an alternate solution — to concentrate his breathing so much that Tanjiro is able to literally will his torn blood vessels into closing through breathing, staunching the bleeding. The Flame Hashira then teases that Tanjiro may be able to do all kinds of things through breathing.
As he gives all of his blood to Tanjiro, Muzan says that if the boy survived this he would become the strongest Demon ever. Since he was the brother of Nezuko Kamado who was the first Demon to develop an immunity to sunlight and also the only one capable of using the Breath Style of Yoriichi Tsugikuni.
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.
The first demon that is said to have existed was Muzan Kibutsuji. The one who turned him into a demon was a generous doctor from the Heian Period, who wanted to save Muzan from death since, at the time, he was diagnosed with a disease which would kill him before he turned twenty.
Moreover, any Nichirin sword can become bright red regardless of the blade's original color. However, the blade can only stay bright red for a certain amount of time before returning to normal.
It is possible for a swordsman to use more than one Breathing Style technique at a time, allowing the user to take advantage of the advantageous aspects of both techniques. Tanjiro Kamado, for example, can use both the Water Breathing and Sun Breathing styles simultaneously.
Not one to back off from a fight nor is he one to overlook injustice; he has fought and slain several demons, but his battle with Daki pushed him over the edge, and with unbridled rage boiling within him, Tanjiro's eyes became bloody.
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.
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.
Because he uses sun breathing, he would be immune to the sun, just like nezuko. Who is the most powerful person in the Demon Slayer universe? Referred to by Inosuke and Tanjiro as the strongest Hashira and Demonslayer.
Chapter 204 of the series reveals the mechanics behind Tanjiro's demon cure, and reveals that a combination of Tamayo's antidote and Nezuko helped Tanjiro out of that predicament.
In the far chapters of the Demon Slayer manga, Tanjiro is actually turned into a demon by Muzan Kibutsuji. Muzan does this to continue his demon legacy and live in this world even after being dead. He chooses Tanjiro over everyone else as he considers him the perfect one to carry on his legacy.
The Thirteenth Form of the Sun Breathing Style is unique in the sense that it doesn't really have a name of its own. Rather, it's a combination of all the twelve forms present in this style, used in succession to maximize their power. This move was devised by Tanjiro solely to defeat Muzan.
the 13th form is the combination of all 12 forms of the sun breathing -_- The user continously performs all twelve forms of the breath style in repetitive succession to increase the accuracy and agility of his movements.
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.
Black. Toho Co., Ltd. Black is the most mysterious color of all the blades, and is held by the main character, Tanjiro. 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.
Gyomei's Gray Flail and Axe
Undoubtedly the strongest of all of the Hashira in terms of pure strength and power, Gyomei uses weapons that are also very unique. While they are still considered Nichirin blades for all intents and purposes, Gyomei's weapons are a flail and an axe.
Tanjiro Kamado wields a black Nichirin blade, but the symbolism of such a sword is unknown. 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.
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.
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.
However, she wasn't immune to the sunlight initially, as she got burned badly at first. So, in conclusion, her coming from a Sun Breathing user's lineage and not relying on human substance are perhaps the reasons for her immunity.