This article contains spoilers for Demon Slayer, by Koyoharu Gotouge, available in English from MangaMonster.
Season 3 of Demon Slayer is getting serious, with clashes against two powerful Upper Moon demons demonstrating why these foes should be dreaded. As a result of these high-stakes battles, Tanjiro and his colleagues are giving it their all, with a couple of the slayers employing the ultimate boost to exterminate the demons.
In the anime's "Swordsmith Village" arc, Tanjiro and the Mist Hashira, Tokito Muichiro, have manifested Demon Slayer Marks, gaining heightened strength and ability to employ against demons. Muichiro used his to beat Upper Moon Gyokko, while Tanjiro's mark appeared for the second time, allowing him to sever the heads of several demons in seconds. This mark appears more frequently in Demon Slayer; here's what it is and the hazards associated with using it.
The Demon Slayer Mark Originates With Yoriichi Tsugikuni
The first to ever manifest the power of the Demon Slayer Mark was the ancient swordsman Yoriichi Tsugikuni, known famously as the strongest demon slayer to have ever existed. As with the rest of his abilities, the mark was viewed as a gift from the heavens or Gods, used to help bring Muzan Kibutsuji's reign of terror to a stop. During the Sengoku Era, many of the demon slayers who fought alongside Yoriichi also manifested the mark, but it is far rarer of an occurrence in the Taishō period when Demon Slayer takes place.
The Demon Slayer Mark is passed from user to user, and as such is believed to be genetic. Both Tanjiro and Nezuko manifested the mark throughout the series' events, and their father also showed signs of it -- even without full understanding or knowledge of it. Alternatively, the original Flame Hashira likely never unlocked his mark, thus no further members of the Rengoku family were able to access the abilities of the marking.
What Does the Demon Slayer Mark Do?
The Demon Slayer Mark is a temporary irregularity that appears on the skin like a birthmark, most commonly on the face or other visible locations of a demon slayer's body. Context from the manga suggests that its main power allows a user to go far beyond their life limit. They can do this freely and without any instant consequences, meaning their increased strength has practically no end. This allows the user to tap into inhuman levels of power and surpass their life limits, fighting when in normal circumstances they should have died.
Another advantage of the mark is a significant improvement in stamina, endurance, and strength. Because the user has no life limit at the time of manifestation, they can battle with grave wounds, such as Muichiro facing Gyokko while the latter was poisoned. Tanjiro, who was previously battling with the varied methods of the Upper Moon Four's emotion demons, wove through three of them at incredible speed and accurately killed everyone in his path after awakening his mark.
The Demon Slayer Mark, on the other hand, is difficult to obtain and will only manifest under certain conditions. An key need is that the user has been in contact with a slayer who has already activated or possesses their mark. Their own mark will never awaken unless they complete this task. The slayer must also be in a life-threatening scenario in order to materialise, specifically a heart rate of more than 200 beats per minute and a body temperature of more than 102° Fahrenheit. In most cases, these conditions would be lethal, but according to Muichiro, the difference between those who are marked and those who never awaken is a strong resolve to life.
The Demon Slayer Mark Has a Major Consequence
With Demon Slayer Mark's major power allowing the wearer to basically dodge death, a high price must be paid. As a result of exceeding their limit, the user's life is doomed to end when they reach the age of 25. Gyomei, the Stone Hashira, characterises this as "paying one's lifespan in advance in exchange for a greater physique" while discussing the mark with Muichiro. A marked demon slayer's lifespan is reduced in return for stronger physical capabilities.
However, there are a handful of exceptions to the Demon Slayer Mark's curse. Nezuko, who activated her mark as a demon, won't be affected by this rule as demon bodies technically have no life limit. That said, if she activated it as a human then she would be subject to the same fate. Similarly, Yoriichi lived past the age of 80, though this exception is more likely due to his already-divine nature, so it couldn't be replicated by a normal human.
The vast majority of users remain subject to the rule, including after the age of 25. Any demon slayer who manifests a mark after turning 25 will not survive the next day, even if it’s their first time using it. This phenomenon is seen with Gyomei, who was 27 when he first accessed the benefits of the mark. Sadly, the Stone Hashira passed away very soon after this.
The Demon Slayer Mark acts as both a blessing and a curse. Since it was first manifested by Yoriichi, it has brought great power to demon slayers -- but at a price. While users can exceed their life limits and tap into exceptional strength, they must sacrifice a large portion of their lifespan to do so.