Bright Red Nichirin Sword

A Bright Red Nichirin Sword refers to the ability that turns a Nichirin Sword bright red with special properties for a certain amount of time and is only accessible by Demon Slayers who have awakened the Demon Slayer Mark.

Overview
Nichirin Swords can turn into virtually any color dependent on the user and their Breathing Style. However, there is one color that cannot be normally accessed: a shining bright red. Unlike other colors, a bright red sword has special properties and can only be accessed by Demon Slayers who have awakened the Demon Slayer Mark. Moreover, any Nichirin sword can become bright red regardless of the blade's original color. However, the blade can only stay bright red for a certain amount of time before returning to normal.

There are three known methods which have been shown to successfully turn Nichirin swords bright red:
 * The first method involves clashing two Nichirin swords together with enough force that the friction causes both blades to turn bright red.
 * The second method involves gripping one's Nichirin sword with so much strength that it raises the blade's temperature from the immense pressure, causing the blade to turn bright red.
 * The third method is the only method which doesn't require a Demon Slayer Mark. It involves covering a Nichirin Sword with Nezuko Kamado's Exploding Blood to raise its temperature to the point it turns bright red.

The most notable effect of a bright red Nichirin sword is its ability to hamper or even completely nullify a demon's regeneration. The bright red Nichirin sword also generates extreme heat, which can cause immense pain to demons and cause them to cough up blood. The extent of which a bright red Nichirin sword can hinder a demon's regenerative ability can be quite apparent, as seen with the likes of Hantengu and Muzan Kibutsuji himself taking around a dozen seconds to regenerate damage that would be healed instantly under normal circumstances.

Bright red Nichirin swords also seem to be able to nullify the regeneration of demons that have just been decapitated, thus permanently killing them even if they managed to regrow their heads after being decapitated by a Nichirin sword. This can be inferred when Kokushibo, who had just overcome his weakness of decapitation from Nichirin swords, slowly crumbled away and was unable to regenerate despite being stabbed in a non-lethal spot by a bright red Nichirin sword.

Yoriichi Tsugikuni's bright red Nichirin sword is the most potent out of all Demon Slayers, being able to burn on a cellular level and leave scars that even the Demon King himself can't heal even after centuries. Furthermore, once slashed by Yoriichi's Nichirin sword, the cells in the affected region will be continuously burned for centuries.

Trivia

 * While the current accepted reading of the Japanese name of the Bright Red Nichirin Sword is "Kakutō", it has also been rendered as "Shakutō" in the past.