Demon Slayer's third anime season, the Swordsmith Village story arc, expanded upon the series' combat systems in a few intriguing ways, from Genya's half-other combat moves to Gyokko's aquatic, vase-themed attacks to the introduction of two more demon slayer marks. These marks are the key to victory against the Upper Moons, but it's not easy getting them.
Earlier, protagonist Tanjiro Kamado awakened his demon slayer mark on his forehead, which some Demon Slayer characters have commented on. Now the love and mist Hashiras, Mitsuri and Muichiro, have awakened theirs as well, and that will totally change how the demon slayer corps will fight Muzan's remaining minions in the next battle, though there are some confounding complications to deal with, too.
The True Nature Of Demon Slayer Marks
Demon slayer marks are a physical mark on a demon slayer's body, proof that the slayer has unlocked incredible power that pushes them far beyond their normal limits. These marks will look different according to a given slayer's breathing style, but all marks look like birthmarks and will appear on a visible part of the body, such as the face, neck, or near the collarbone. In a way, these demon slayer marks represent supernatural marks found on demons, such as Nezuko Kamado's full-body marks in her berserker state, though these demon slayer marks don't actually have a connection to demonic powers. They are not a half-other power, though they do grant the strength needed to rival or even surpass most demons.
A demon slayer mark will appear if a given slayer experiences and endures a state of extreme distress where their very survival is threatened, and the mist Hashira, Muichiro Tokito, took note of that. During his fight against Gyokko, Muichiro nearly drowned in Gyokko's water-themed attacks, but Muichiro survived despite his body's suffering. Muichiro also had a heartbeat in excess of 200 beats per minute, and his body temperature became feverishly high. Muichiro's mark, which appeared on his cheeks, gave him the strength needed to defeat Gyokko's final form with shocking ease, and elsewhere, the love Hashira Mitsuri Kanroji awakened her own mark to even the odds against Hantengu's powerful youth form.
Overall, demon slayer marks are a typical shonen-style power-up in Demon Slayer, and they are straightforward compared to the intricacies of the well-established breathing styles such as water breathing or Hinokami Kagura. This is how demon slayers go Super Saiyan, and given how tough the Upper Moons are and the demon slayer corps' constant state of desperation, this is exactly what they need to face Muzan and the three remaining Upper Moons in future battles. Even if it feels a bit convenient and routine, demon slayer marks are the key to victory – but actually harnessing this power, rather than just stumbling upon it, will be the real challenge, one that the Hashira are already getting ready for.
The Hashira Must Train All Over Again To Harness Their Demon Slayer Marks
So far, even the elite Hashira only barely win their fights against the Upper Moons, if they even win at all. Kyojuro Rengoku died at Akaza's hands, while Tengen Uzui was injured to the point of retiring after his ordeal against Gyutaro, the Upper Moon 6. Mitsuri Kanroji might have died if it weren't for Tanjiro's timely slaying of Hantengu's main body, and while Muichiro had a relatively easy time, he still struggled mightily and almost died fighting Gyokko. Muzan Kibutsuji the demon king and the remaining three Upper Moons are even stronger than that, and only seven Hashira remain. Those seven must re-enter training mode and act like students rather than masters, putting them in the same situation as Tanjiro and other journeyman demon slayers. They're all human, Hashira or not, and all humans have room to grow and improve.
Demon slayer marks represent the last way in which the elite Hashira can boost their skills and up their game before facing the final four villains in future Demon Slayer seasons and movies. Their swordplay is already sufficient, so now the Hashira must focus on marks, and even for them, this won't be easy. Mitsuri and Muichiro, and even Tanjiro Kamado for that matter, only awakened their marks through extreme circumstances of desperate survival at all costs, and it's not easy to replicate those conditions outside of battle. The Hashira might awaken their marks while facing Akaza, Doma, and Kokushibo in battle, but there's no guarantee that will work in time to ensure victory. Instead, the Hashira must master and be able to summon their marks on demand.
This may become a major plot point in Season 4 and beyond, and it opens up new and exciting possibilities for Demon Slayer's narrative. Even if the "train to unlock new powers" trope is familiar, it should still be fun to watch the mighty Hashira humble themselves and become students in training once again. This may reveal a lot about them not just as fighters, but as people too, and how they respond to the hardships and challenges of mark training should add much-needed depth to these characters. Mitsuri and Muichiro have a head start, though, and that may make them the demon slayer corps' most prominent and helpful members during this critical time.