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.
Nichirin Indigo Blade
The color indigo is related to another tier of breath skills. The Breath of the Beast, a derivative of the Breath of Wind fighting style was developed by the indigo sword wielder, Insouke Hashibira. With eleven focuses, Breath of the Beast was created for survival in distant places.