Nichirin Swords (日にち輪りん刀とう Nichirin Tō?) are special blades used by and made specifically for the Demon Slayer Corps to slay demons. They are forged from a unique ore that constantly absorbs sunlight, one of the only major weaknesses of Demons. Therefore, cutting the neck-bone with a Nichirin Blade is one of the few ways a human can kill a demon, the other being the use of a Wisteria-poison coated weapon.[1]
Nichirin Swords take on a distinct color when first drawn by its owner, which is why they are also referred to as "Color Changing Katanas".[2] However, the holder is required to have a certain amount of skill, or the blade will not change color.