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. It resembles the Demon Crests that Demons possess. It looks somewhat like a tattoo, scar or birthmark and each Demon Slayer Mark looks unique related to the Breath Style of the bearer. However, the precondition in unlocking it is that an individual directly related to the Breath of the Sun has been born with it, after which it will then spread to other strong Demon Slayers through resonance.

History
The true origin of the Demon Slayer Mark is unknown. However, it is known that the first person with a Demon Slayer Mark was none other than the First Breath user, Yoriichi Tsugikuni. He was born with the mark and it enabled him as a child to gaze into the "see through world", allowing him to see the disease within his mother, and also to defeat strong opponents by seeing their blood flow, muscle and joint movement. 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" Demon Slayer Corps that almost succeeded in killing Muzan Kibutsuji. Demon Slayer Marks have 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 other Pillars that 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. 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 Pillars. Each Demon Slayer Mark has a unique appearance that is related to the user's Breath Style. For unknown reasons, Yoriichi Tsugikuni was born with a Demon Slayer Mark.

Abilities

 * Enhanced Physical Capabilities: Upon activating the mark, the user is granted superhuman strength, speed and enhanced breathing techniques beyond a normal Breath Style user. Their strength and speed could even rival that of stronger Demons like the Upper Moons of the Twelve Demon Moons.


 * See-Through World: A ability that can be unlocked by Demon Slayer Mark Users. It enables the user to see the muscles, blood flow, and joint movement of living beings, allowing them either to accurately predict and react to movements or confirm their well-being. Additionally, the user is able to perceive fast movements as slow movements in their eyes.


 * Crimson Red Nichirin Blade: Demon Slayer Marks may enable the users to dye their Nichirin Blade in a crimson red colour, giving their blades the ability to hamper the enhanced regeneration of demons.

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 after they reach the age of 25, which Kokushibo speculated to be because the mark exchanged the life span of its user for power. However, there are ways to avoid dying through the curse as shown by Yoriichi and Kokushibo. Through unknown means, Yoriichi was able to defy fate and lived up to 85 years, while Kokushibo choose to become immortal by becoming a Demon. However, all records of users of the Mark dying at the age of 25 are of much older era than present times, which means this can be merely a superstition, as many Demon Slayers die at a young age because of their dangerous lifestyle (as it was seen many times that activating the mark won't necessarily allow the user to win every single fight).

Known Users and their pattern

 * Yoriichi Tsugikuni, with a flame like pattern on the left side of his forehead.
 * Kokushibo, with a flame like pattern on the right side of his lower cheek and one on the left side of his forehead.
 * Tanjiro Kamado, with a scar looking like birthmark on his left side of his forehead like his father at the beginning. After getting injured in the Final Selection, it starts looking like a big scratch which later further grew into a flame-like pattern, resembling Yoriichi's and Kokushibo's.
 * Muichiro Tokito, with mist cloud patterns on both sides of his cheeks and one on the left side of his forehead.
 * Mitsuri Kanroji, with a double heart pattern above her left clavicle.
 * Giyu Tomioka, with a water stream like pattern on the left side of his cheek.
 * Gyomei Himejima, with fissure-like patterns on both of his forearms.
 * Sanemi Shinazugawa, with a windmill resembling pattern on the right side of his cheek.
 * Obanai Iguro, with a patterm resembling a snake around his left arm.