Demon Slayer Mark

The Demon Slayer Mark is a mysterious marking that can be unlocked and may appear on the body of a strong Demon Slayer.

Overview
The most talented Demon Slayers of the Demon Slayer Corps have a chance to develop a mysterious tattoo, scar, or birthmark marking. It resembles the crests that demons possess. These marks are referred to as Demon Slayer Marks. Each Demon Slayer Mark looks unique, relating to the Breathing Style of the bearer.

The true origin of the Demon Slayer Mark is unknown, however, the first recorded occurrence of these markings were in the Sengoku Era, possessed by the first person with a Demon Slayer Mark was none other than the first Breathing Style user, Yoriichi Tsugikuni, who, for unknown reasons, was born with a Demon Slayer Mark. Practically all other swordsmen during the Sengoku Era awakened their Demon Slayer Mark due to the catalyst-like effect of the Mark.

It is said that the Demon Slayer Mark has resonated since then and has been awakened by other Demon Slayers, which would later lay the foundation for the "Golden Era" of the Demon Slayer Corps, a time when the Demon Slayers came within inches of killing Muzan Kibutsuji.

Demon Slayer Marks disappeared for a long time, due to the countless times the Demon Slayer Corps was nearly annihilated, until it reappeared on Tanjiro Kamado, and subsequently, the Hashira who resonated with his mark.

Unlocking
The Demon Slayer Mark can only appear in Demon Slayers that survived the life-threatening conditions of having a heart rate over 200 bpm and a body temperature over 39° Celsius (102.2° Fahrenheit). The precondition in unlocking it is that an individual directly related to Sun Breathing has been born with it.

The Demon Slayer's mark also has the ability to act as a catalyst, and spread from the original user to other strong Slayers, such as the Hashira, through resonance, granting them Demon Slayer Marks of their own.

Abilities
Bright Red Nichirin Sword: Demon Slayer Marks may enable the users to dye their Nichirin Sword a bright red color, giving their blades the ability to hamper the enhanced regeneration of demons.

Enhanced Physical Capabilities: Upon activating the Mark, the user is granted superhuman strength, speed, and enhanced breathing techniques beyond that of a normal Breathing Style user. Their strength and speed could even rival that of the strongest demons, such as the Upper Ranks of the Twelve Kizuki.

Transparent World: The user becomes able to see the muscles, blood flow, and joint movement of living beings, allowing the user to accurately predict and react to their opponent's movements, as well as confirm the health and well-being of others. Additionally, the user gains hyper-perception, allowing them to perceive the world around them at a much fast rate, making it appear to them as if the world was going in slow motion.

Curse of the Mark
Having the Demon Slayer Mark has its costs. Demon Slayers who have awakened the Demon Slayer Mark are said to die upon reaching the age of 25. Kokushibo mentioned that despite the increased power, it reduces their life span due to the strain on their bodies as previously stated by Shinobu, who stated the conditions for unlocking it alone was deadly even for a Hashira. In a conversation Muichiro Tokito had with Gyomei Himejima, Gyomei describes the curse’s effect as "paying one's lifespan in advance in the exchange for a greater physique".

There are ways to avoid death through the curse, as shown by Yoriichi and Kokushibo. Through unknown means, Yoriichi was able to live to be 85 years old, however, this could be due to being born with it unlike others who had to awaken it. Kokushibo, on the other hand, avoided dying from the curse by becoming immortal as a result of turning into a Demon.

It is still possible to awaken the mark after a Demon Slayer is older than 25, evident in Gyomei activating it and receiving the benefits without dying immediately. Kokushibo surmised that due to this unique situation, Gyomei would die within a few hours of activating it. Though Kokushibo's prediction came true, it is unknown if Gyomei's death was a result of the curse or his injuries from the final battle against Muzan.

Due to the curse's inconsistency, it is possible that it's simply superstition. However, there is nothing that comprehensively disproves it either, so it's just as likely that the curse does truly exist.