Nichirin Blades are special blades used by and made specifically for the Demon Slaying Corps to slay Demons. They are forged from an unique ore that constantly absorbs sunlight, one of demons only major weaknesses. Therefore, decapitation by Nichirin Blades is the only way a human can kill a Demon, besides 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".
Nichirin Blades are forged with Scarlet Crimson Iron Sand and Scarlet Crimson Ore, both of which are found on high mountains like the Sunlight Mountain, which is perpetually bathed in sunlight all throughout the year.
Demon Slayer recruits choose their own ore, after successfully attending the Final Selection. The ore then will be used by a swordsmith of the Swordsmith Village during the forging process, who will then bring the weapon personally to the Demon Slayer.
Nichirin Blades change color depending upon the user, making each blade unique. They can be of many colors. Zenitsu Agatsuma's blade is unique in that its coloration does not form as a solid band but instead forms a lightning motif down the length of the blade. The reason for this patterning is unknown. Each color is said to impart certain properties to the blade. The properties of black Nichirin Blades are not well understood, causing them to be considered a bad omen that the Demon Slayer will not go far (due to dying before much can be learned about the them).