Demon Slayer as a story has been relatively grounded in its power system when it comes to the Demon Slayers. Excluding the demons, Demon Slayers are simply human beings with extraordinary breathing techniques and swordsmanship that allow them to push their bodies to their limits. This defines much of the dynamic behind every battle between Tanjiro and the demon's he has to defeat. However, eventually, the threats of Demons became too great for any one Demon Slayer to handle on their own. Demon Slayer: The Entertainment District Arc shows this, where for the majority of the season Tanjiro, Zenitsu, Inosuke, and Tengen Uzui combat against Upper Moon 6 Daki and Gyutaro in a climactic battle. However, near the end of that season we are given a glimpse of a Demon Slayer mark that appeared over Tanjiro's scar. This meant the victory for the Demon Slayers, but it wouldn't be until season 3 where we truly get a closer look at these marks and especially their effects on the Hashira.
Over the course of season 3, Tanjiro, Mitsuri, and Muichiro confronted and manifested their own demon slayer marks during the fights against the Upper Moon's 5 and 4. This meant that the three of them had a greater chance to keep up with the Demons, giving way to one animation marvel of a scene where Tanjiro manages to slice the head off of three demon clones at the same time. And with Muichiro, his mark seemed to make his battle against the Upper Moon 5 nearly paltry work once he managed to manifest it, due to being a natural genius. And with Mitsuri, the Love Hashira, she was able to keep up with Zohakuten despite being on the defensive, but most likely was able to survive thanks to manifesting the mark in the first place. As to the ability to make these marks appear and why they are important, that leaves many people theorizing, but there is potentially a good reasonable explanation for this.
The Effects Of The Demon Slayer Mark
As early as episode 10, season 2 of Demon Slayer; the Demon Slayer Mark made its appearance on Tanjiro's forehead during the critical moment of beheading Gyutaro and Daki at the same time. However, Tanjiro described it as pushing himself to beyond his limits, and it succeeded in allowing him to decapitate Gyutaro while Zenitsu and Inosuke beheaded Daki. While the effects were difficult to note at the time, with season 3 it is now known that the Demon Slayer Mark manifesting means that the wielder gains enhanced speed, and physical capabilities are exponentially increased by a wide margin. And this ability is shown to not be exclusive to Tanjiro, as the Mist and Love Hashira respectively managed to manifest the same Demon Slayer Mark that increased their abilities and allowed them to combat the Upper Moons on a more even playing field.
In the case of Muichiro, who had been struggling against Upper Moon 5 Gyokko for the first half of the arc, when his mark manifested he began stomping through Gyokko's attacks and even defeating him single-handedly with nearly no struggle or issues aside from the needles from before manifesting the mark. And while Mitsuri was the last to manifest the Demon Slayer Mark, she manifested it in the most critical moments when combating Zohakuten. This allowed her to survive longer and hold off the demon for longer until Tanjiro had a chance to decapitate Upper Moon 4 Hantengu. This demonstration of the physical skills that three slayers were able to perform under the Demon Slayer's Mark is enough to provide a new skill to the repertoire of techniques that the main characters will need to combat against the rest of the Upper Moons.
What This Means And Why
While fighting against the Upper Moons is by no means an easy feat, especially fighting one until sun-rise, the Demon Slayer Mark allows the heroes to close the gap of strength between human and demon; which has its own set of mysteries that have yet to be revealed or even made a connection to the first Sun Breather. This Demon Slayer Mark means that the heroes have a new power boost that will allow them to have a higher chance of surviving and defeating some of the Demons that they are most likely going to face, such as Akaza, Douma, and Kokushibo. This is mere speculation however, and there is much more that needs to be learned about the red-blade that Tanjiro used against Upper Moon 4, and the Demon Slayer's mark has vague and sometimes inconsistent requirements to achieve, so we're not sure if even Inosuke or Zenitsu will achieve the Demon Slayer Mark.
The thing is, this importance allows for the fights to have a narrative equality between the villains and the good guys. With higher stakes, comes a need for higher power and a sense of danger for both sides for the story to grasp our attention. Demon Slayer manages to do so explosively well thanks to the Demon Slayer Mark's introduction and usage throughout one arc.
Demon Slayer is available to stream on Crunchyroll.