I realized when I watched demon slayer that both Tanjiro and his dad both have scars on their foreheads, and I don't know if this applied to his dad but Tanjiros scar is always changing shape and size. Does tanjiros forehead scar have to do with anything about Tanjiro's dad being able to use the fire god style? If you have any info on Tanjiros scar id also like to know that.
Tanjiro told that, although the scar looked like a demon hunter mark, it hadn't been one until he mastered the mark, as wiki summarizes it:
He also has a scar on the top left of his forehead, which he received when he protected his younger brother from a falling brazier. The scar originally looked somewhat like a graze until his fight with the Hand Demon during the Final Selection, after which it develops into his Demon Slayer Mark.