Demon Slayer Mark

The Demon Slayer Mark is a mysterious marking that Demon Slayers can awaken under specific conditions.

Overview
Demon Slayer Marks are irregularities on a Demon Slayer's skin that resembles a birthmark. It commonly appears on the face or another visible location of the body of Demon Slayers that resemble the crests of demons. Each Demon Slayer possesses a unique pattern to their Mark which appear to be based on the Breathing Style they utilize; the only known exceptions are Tanjiro Kamado, Yoriichi Tsugikuni, and Kokushibo, all three of which have Demon Slayer Marks resembling flames that manifest on their foreheads. The Mark can only be awakened under very specific conditions and grants special abilities while also enhancing pre-existing traits of the bearer. Once awakened, the Mark can disappear after a period of time along with its effects, though it can be re-activated (as seen with Muichiro Tokito and Tanjiro Kamado) by meeting the same conditions, or through training to maintain the "Marked State" to prevent it from dissipating in the first place. One special property of the Mark is that it is able to spread from one bearer to the other as if they were resonating which each other.

Tanjiro Kamado's Demon Slayer Marks has been shown to increase in length repeatedly after awakening it, although it is unknown if this trait is exclusive to him or if it also manifests in other Demon Slayers.

Unlocking
For one to awaken the Demon Slayer Mark, they must survive extremely life-threatening conditions, specifically a heart rate of over 200 beats per minute and a body temperature of over 39° Celsius (102.2° Fahrenheit), conditions which can prove to be fatal in most cases. Muichiro Tokito states that survival is the key difference that separates those that are marked and those who fail to awaken it.

However, perhaps the most important prerequisite is being in contact with an individual that possesses the Mark or has been marked before. Without fulfilling this condition, it cannot be awakened at all.

Origin
During the Sengoku Era, the Golden Age of Demon Slayers, the progenitors of the first ever Breathing Styles all manifested markings on their bodies, which later came to be known as Demon Slayer Marks. The first wielder of the Demon Slayer Mark is Yoriichi Tsugikuni, the strongest Demon Slayer in existence as well as the progenitor of Breathing Styles. Unlike other Demon Slayers that have to meet specific conditions to activate the Mark, Yoriichi was born with it. Upon joining the Demon Slayer Corps, Yoriichi spread the Mark to all other swordsmen in the organization.

Despite how common the Mark was among the Demon Slayers during that era, most records of the Mark were lost to time either because the bearers then did not think preserving the information was important and/or because the information was lost in the many times the Demon Slayer Corps was nearly annihilated.

Abilities
Bright Red Nichirin Sword: Bearers of the Demon Slayer Mark can dye their Nichirin Swords a shining bright red color mainly through clashing their blade with another marked swordsman's or gripping it with tremendous force. Bright Red Nichirin Swords have the ability to generate intense heat and also hamper the regeneration of demons, some of which on the level of the strongest Upper Ranks of the Twelve Kizuki and the Demon King himself.

Enhanced Physical Capabilities: Upon awakening the Demon Slayer Mark, the physical strength and speed of the bearer is exponentially increased to the point where Hashira or Hashira-level Demon Slayers are able to overpower and defeat the Upper Ranks of the Twelve Kizuki and grow in power comparable to the highest among them. It is implied that the bearer's physical arm strength is increased by around 100x after awakening the mark, though it could be a hyperbole and thus the value could be much lower. The increase in the speed of the bearer is quite apparent as well, as seen when Muichiro Tokito, who was initially having trouble dealing with Gyokko's various techniques, became capable of regularly out-speeding and even blitzing Gyokko in his true form after activating his mark.

Enhanced Resistance: It is implied that the Demon Slayer Mark increases the bearer's resistance to Muzan Kibutsuji's demon-transforming blood, much like Total Concentration Breathing.

Transparent World: Bearers of the Demon Slayer Mark can potentially gain access and see into the Transparent World, allowing them to perceive the bodies of others as if their skin was transparent. This enables them to track the flow of blood, breathing, muscular contractions and joint movements of the person they are looking at, which can be used to predict their movements and evade their attacks with ease. Additionally, those who can access the Transparent World can perceive the world in slow-motion, further increasing their reaction and movement speed.

Curse of the Demon Slayer Mark
Awakening the Demon Slayer Mark comes at a steep price. Demon Slayers who have awakened their mark are said to, without exception, die before reaching the age of 25. Kokushibo states that awakening the mark merely reduces the lifespans of their bearers in exchange for greater power, while Gyomei Himejima describes it as "paying one's lifespan in advance in the exchange for a greater physique." However, Kokushibo managed to survive through accepting Muzan Kibutsuji's offer to become a demon, living for roughly another four centuries after aging past 25, indicating that gaining eternal life through demonification allows one to cheat the mark's curse.

Despite what the curse entails, it is still possible to awaken the mark after a Demon Slayer is older than 25, evident in Gyomei awakening it along with gaining its benefits. Kokushibo surmised that due to this unique situation, Gyomei would die within a few hours of activating it, which came to pass after the Demon Slayer Corps' final battle against Muzan, though it is possible that he died due to the injuries he sustained during the battle rather than the curse itself. Furthermore, Yoriichi Tsugikuni, the first user of the mark, managed to live past the age of 80, though this can also be explained by the fact that he was born with it.

Trivia

 * Kokushibo is the only known demon to possess the Demon Slayer Mark, due to having it before he transformed.

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