Bright Red Nichirin Sword

A Bright Red Nichirin Blade (く) is a special technique that can only be accessed by users of the Demon Slayer Mark.

Overview
Nichirin Blades can turn into virtually any color dependent on the user and their Breathing Style, however, there is one color that cannot be accessed, which is a crimson red color. This color was first discovered by Yoriichi Tsugikuni when he became a Demon Slayer due to his Sun Breathing. In fact, it is a category of Yoriichi's gifted powers and abilities, which is meant to hamper the regeneration of demons.

Unlocking
The bright red blade color could only be unlocked by those with Demon Slayer Marks, as well as incredibly strong Demon Slayers, like the Hashira.

The method which it is accessed varies between known users, for Tanjiro Kamado and Yoriichi Tsugikuni, it naturally activated when they engaged in combat, though the former discovered it through Nezuko Kamado setting his katana ablaze and has more trouble in turning it red, especially after his partial demonification by Muzan Kibutsuji's hand. Some of the known users, namely Muichiro Tokito and Obanai Iguro, they accessed it through focusing all of their strength and power on their katana, using the "grip strength of a vise" as described by Obanai Iguro, which he surmised from Muichiro Tokito himself. The other users accessed it through the clashing of their weapons, the friction, heat combined with their superhuman strength and Demon Slayer Mark, turned their Nichirin Blade crimson red.

Abilities
Demonic Regeneration Hampering: The main and only special ability of the bright red Nichirin blade is to hamper or nullify the regenerative and healing abilities of demons, which is meant to mimic the sun's effect on them. This even slowed down the instant regeneration of the Demon King, Muzan Kibutsuji, to the point it took several seconds to heal a sliced off arm, which would heal in less than a split second under normal circumstances. When Muichiro Tokito stabbed Kokushibo with his bright red katana, Kokushibo described it as "searing his insides" which dealt an incredibly significant damage to the Upper Rank, and even caused his flesh to crumble away in the spot that Muichiro stabbed. The searing effect of the bright red Nichirin blade seems to mimic the effect of sunlight on the bodies of demons, burning and scarring them on a cellular level that doesn't fully heal even after centuries of regeneration.