Demon Slayer Mark

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

Overview
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.

Demon Slayer Marks are birthmarks that appear on the face (or sometimes on 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 Mark can only be awakened under very specific conditions and grant 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.

Although this could only be the case for Tanjiro Kamado, the Demon Slayer Mark has been shown to grow in length and size after repeatedly awakening it, which can possibly improve how much the Mark enhances the bearer's abilities.

Unlocking
For one to awaken the Mark, one of the criteria they have to meet is surviving extremely life-threatening conditions, specifically, a heart rate of over 200 beats per minute (BPM) 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, the Mark cannot be awakened at all.

Origin
The progenitor of the 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, enabling them to awaken their own Marks as well.

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 and the Demon King himself.

Enhanced Physical Capabilities: Upon awakening the 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 lowest of the Upper Ranks and fight on equal ground 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 with Muichiro Tokito who was having trouble dealing with Upper Rank Five's various techniques and later being capable of regularly out-speeding and even blitzing him in Gyokko's awakened form after Muichiro activated his Mark.

Enhanced Resistance: It is implied that the Mark increases the bearer's resistance to Muzan's demon-transforming blood similar to how Total Concentration Breathing does.

Transparent World: Bearers of the 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 Mark
Awakening the Demon Slayer Mark also has its costs. 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 describes it as "paying one's lifespan in advance in the exchange for a greater physique" in a conversation with Muichiro.

However, Kokushibo managed to survive through accepting Muzan's offer to become a demon, living for roughly another four centuries after aging past 25, suggesting 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 final fight 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 managed to live past the age of 80, though this can also be explained by the fact that he was born with the Mark.

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.

Trivia

 * Due to only seeing one Mark awakening in the anime at this point, it is currently unclear whether the Demon Slayer Mark can also change the shape of the bearer's hair to match their Breathing Style as seen with Tanjiro.
 * One possible explanation for the Curse of the Mark is that the exponential increase in power the bearer experiences upon awakening their Demon Slayer Mark takes a heavy toll on their body, the strain resulting in a decreased lifespan which is supported by Shinobu's remark that the conditions for awakening the Mark is harmful to the body. This also explains why once the Mark is awakened, there is no way to gain back the forfeited lifespan.