Bright Red Nichirin Sword

First appearing in Chapter 112 and the forty-ninth anime episode, the Bright Red Nichirin Sword refers to an effect that briefly tints a Nichirin blade a vivid crimson while granting it unusual abilities. A Nichirin blade can ordinarily shift to almost any color based on its owner and chosen Breathing Style, but a shining red is the one shade that cannot be reached through normal means. Reaching it generally requires a swordsman whose Demon Slayer Mark has awakened, and any blade can take on the hue no matter its original color. The transformation lasts only a limited stretch of time before the blade reverts. Kokushibo's surprise at seeing red blades from fighters who do not use Sun Breathing suggests the phenomenon was thought to belong to that breathing line alone.

Methods of activation

The crimson glow comes from raising the blade's temperature, converting motion into heat. Three approaches are documented. The first is friction, where two swords are struck together hard enough that the heat from the impact sets both blades alight in red. The second is pressure, gripping the weapon with such unrelenting force that the concentrated strain heats the metal until it reddens. The third is ignition, in which Nezuko Kamado's combustible flames wrap the blade and drive its temperature to an extreme; this is the lone method that does not require the Demon Slayer Mark.

Effects

The signature trait of a red blade is its power to slow or fully shut down a demon's regeneration. The intense heat it radiates causes demons severe pain and can make them cough up blood. Foes who normally heal instantly, including Hantengu and Muzan Kibutsuji, have been forced to spend roughly a dozen seconds mending wounds while cut by a red blade. It can also lock out the recovery of a freshly decapitated demon, sealing a kill that might otherwise be undone, as happened when Kokushibo crumbled despite only a non-lethal stab. Potency differs between users; Yoriichi Tsugikuni's red blade was the strongest of all, burning at the cellular level and leaving wounds the Demon King could not heal even after centuries.

The known wielders are Yoriichi Tsugikuni, Tanjiro Kamado, Muichiro Tokito, Gyomei Himejima, Sanemi Shinazugawa, Obanai Iguro, and Giyu Tomioka. In recorded instances, Yoriichi and Muichiro reddened their blades through pressure, while Gyomei, Sanemi, and Giyu used friction by clashing weapons. Nezuko Kamado triggered the effect on Tanjiro's katana through ignition, the only activation that bypasses the Mark requirement. Tanjiro and Giyu later forced the transformation through pressure while holding Muzan down during the final battle.