Nichirin Sword

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.

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 passing the Final Selection. The ore will then be used by a swordsmith of the Swordsmith Village during the forging process, and the swordsmith will personally bring the weapon to the Demon Slayer.

Blade Colors
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 as 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 blade). For unknown reasons, some Demon Slayers are able to change their Nichirin Blade's color to red, which prevents demons from regenerating damage done by the blade. Clashing a red Nichirin Blade with other Nichirin Blades will also turn them red and give them the same qualities.