There are plenty of things that make Demon Slayers special when compared to the rest of humanity. The intense training process members of the corps go through prepares them for the grueling task ahead of them while honing skills most never tap into. As with anything else, some Demon Slayers are more adept than others.
The Demon Slayer Mark is something only the strongest Demon Slayers can unlock. This rare marking doesn’t show up very often in the Demon Slayer series and lore, which makes it a bit of a mystery. Only a select group of characters in the series have been shown to possess it, putting its unique properties on full display.
How It’s Obtained
There are two kinds of people who can unlock the Demon Slayer Mark; those who survive having a heart rate over 200 BPM as well as having their body temperature reach over 39° Celsius, and those who have a relation to Sun Breathing. Demon Slayers who meet either of these requirements then gain access to a handful of incredibly powerful and useful abilities.
In certain cases, Demon Slayers can also spread their mark to others, should they themselves be strong enough to receive it. The Hashira are typically the only members of the corps that can handle such strain, and plenty of the current group managed to gain some form of the mark during the narrative. More than half of the Hashira were able to unlock their own unique Demon Slayer Marks, showing their resilience and providing the corps with an incredible advantage in the battle against Muzan and his forces.
Once the mark is unlocked, Demon Slayers can make use of some incredibly effective abilities that greatly aid in their efforts to combat demons. Breathing style users already posses incredible strength and speed, but the Demon Slayer Mark greatly amplifies these powers while also giving them access to superior breathing techniques. The mark also unlocks the use of the Red Nichirin Sword ability, which is a devastating technique that can slay even the strongest of the Twelve Kizuki through harnessing the power of the sun.
The most beneficial of these is the Transparent World ability, something even fewer Slayers have been able to unlock. By utilizing this technique, users are able to predict their opponents’ movements, giving them the edge in fights against particularly powerful demons. This helps close the gap in speed and strength between deadly demons like Muzan and Kokushibo, and combined with the other abilities granted by the mark, exposes the typically invulnerable demons.
The One Major Drawback
As with any powerful ability, the Demon Slayer Mark comes at a price. The incredible circumstances it takes for most to obtain the mark, alongside the strain put on the body when using it apparently makes it so that most users don’t live past the age of 25 when they do. This doesn’t seem to be the case for all involved, as there are plenty of notable examples of those who have survived despite possessing the Demon Slayer Mark.
Yoriichi, who is the first known Demon Slayer to have the mark, lived well into his 80s despite regularly making use of its abilities. In his case, it’s possible that because he was born with it due to his natural proficiency with Sun Breathing, his body was not put under the same level of strain as those who had to put in more effort to bring theirs out.
Yoriichi’s brother, Kokushibo managed to circumvent the curse by turning into a demon. So while the mark and its perks remained, the apparent cost of utilizing it was neutralized by his immortality. The curse’s inconsistency can stem from the varying ways in which the mark is acquired. Those who are born with the mark seem to have a higher tolerance for the strain put on their bodies while those who acquire it later in life are more likely to perish from the fatigue it brings on.