Nichirin Blades are special blades used by and made specifically for the Demon Slaying Corps to slay Demons. They are forged from a unique ore that constantly absorbs sunlight, one of the only major weaknesses of Demons. Therefore, decapitation by Nichirin Blade is one of the only ways a human can kill a Demon, the other being the use of a Wisteria-poison coated weapon.
Nichirin Blades take on a distinct color when first drawn by its owner, which is why they are also referred to as "The Color Changing Swords". However, the holder is required to have a certain amount of skill, or the blade will not change color.
The red Nichirin blade, wielded by Kyojuro Rengoku, symbolizes flame. The brightly-burning Flame Pillar is wonderfully charismatic and often cheerfully eccentric. He has a strong sense of morals and his passion for demon slaying burns with the intense heat of a raging flame.
Like Zenitsu's blade, Kyojuro's blade color took the form of a unique pattern. The red color of his blade manifested in a flame motif that runs down the length of his blade that looks spectacular in conjunction with his Breath of Flames style.