WARNING: The following contains spoilers for Demon Slayer: Kimetsu no Yaiba Season 2, Episode 13, "Layered Memories," now streaming on Funimation, Hulu and Crunchyroll.
Tanjiro's scar has been the subject of debate and confusion for fans throughout the history of Demon Slayer: Kimetsu no Yaiba. For the second time in the "Entertainment District" arc, Shinjuro Rengoku, father of the late Kyojuro, has unceremoniously answered one of the anime's most burning (no pun intended) questions. As it turns out, the scar on Tanjiro's forehead does have some significance to it, rather than simply being a Todoroki-esque burn from scalding water.
Tanjiro was not born with this scar on his head; rather, he acquired it when tackling his younger brother out of the way of a falling pot of boiling tea. At the beginning of the series, the scar was a light pink circular mark on his head. The only noteworthy thing about his scar in the series' early days was that, despite it being the result of an accident and not a birthmark, his father bore a near-identical mark in the same spot on his own forehead. Nonetheless, his father's scar was never addressed, and the series was apparently content to leave it as a simple coincidence.
It wasn't until the Final Selection that Tanjiro's scar started to get… well, weird. After suffering a blow to his head directly where his scar is located, Tanjiro's head is bloodied or bandaged for the next several episodes. However, when he eventually takes the bandages off, the scar is completely different -- now a jagged, dark red line. However, this change was of interest to exactly no one within the series, leaving the audience to assume the change was simply a retcon to make the mark look cooler.
Fast forward all the way to the "Entertainment District" arc, and Tanjiro's scar is now a hot topic of conversation once more. During his battle with Daki, Upper-Rank Six of the Twelve Kizuki, Tanjiro recalled a letter he received from Shinjuro Rengoku in the time since their first, unpleasant encounter. Shinjuro seems to be the only person who knows anything at all about Sun-Breathing, and it's in this letter that he explained to Tanjiro that the scar on his head is the mark of one who has been chosen to wield this technique.
However, Shinjuro may not have all the facts right, as he claims this mark is something people are born with, which Tanjiro assuredly was not. This alone would be enough to refute Shinjuro's claim, but when a memory from Muzan rises out of Daki's subconscious, it is a vision of a man wearing Hanafuda Earrings -- the symbol of Sun-Breathing -- and sporting a scar nearly identical to Tanjiro's. It is also noted that Tanjiro's father, who taught Tanjiro the various forms of Sun-Breathing, was born with a scar on his head.
Ultimately, too much is lining up for Shinjuro's claim to be anything but the truth. Sun-Breathing appears to be a thing of destiny rather than a Breathing Style anyone can learn. As the original and most powerful of all Breathing Styles, Sun-Breathing might just be the only thing capable of truly harming Muzan Kibutsuji and his most powerful demons. Judging by the visceral reaction of Muzan's cells to the memory brought on by Tanjiro's uncanny resemblance to the mysterious Sun-Breather, the Progenitor of Demons is not a fan of Sun-Breathing. All Tanjiro has to do now is stay alive long enough to master the Hinokami Kagura, because Muzan already wants no part of him even though Tanjiro can barely survive Sun-Breathing's full power.
