Nichirin Sword

Nichirin Blades are a special blade used by and made specifically for the Demon Slaying Corps to slay Demons.They are made of an ore that constantly absorbs sunlight, a demons only weakness. Therefore, decapitation by Nichirin Blades is the only way a human can kill a demon.

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 Sword".

Forging Process
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 for the forging process, which will later bring the weapon personally to the Demon Slayer.

Blade Colors
Nichirin Blades change colour depending upon the user, making each blade unique. They can be of many colors. Zenitsu'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 an omen that the demon hunter will not go far (due to dying before mich can be learned about the hue).