Nichirin Sword

Nichirin Swords 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 only ways a human can kill a Demon, the other being the use of a Wisteria-poison coated weapon.

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". However, the holder is required to have a certain amount of skill, or the blade will not change color.

Forging Process
Nichirin Swords 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 on Mt. Yoko.

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.

Handguards
Each Nichirin Sword contains a handguard, or tsuba, that protects the swordsman from cutting themselves when they have a loose grip. It is located where the handle and the blade meet and can come in a variety of colors and shapes. Based on his appearances in the series, Inosuke Hashibira's Nichirin Swords temporarily had handguards when Kozo Kanamori first presented his newly-forged swords. Afterwards, he made his swords jagged-edged, removed the handguards, and wrapped the hilt with bandages.

*Tanjiro began using Kyojuro's handguard as of Chapter 129.

*Inosuke removed the handguard shortly after receiving the reforged sword.

*Although Kaigaku replaced his Nichirin Sword for one constructed of Demon flesh, his new sword appears to have the same handle and tsuba as his old one.

Sword Colors
Nichirin Swords 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 Swords 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).

Bright Red Nichirin Swords
By raising the temperature of the blade, a Nichirin blade will turn red. Bright Red Nichirin Swords hamper the regeneration of demons and in some cases even burn them on a cellular level leaving wounds that are unable to heal. The creation of the Bright Red Nichirin swords is the transfer of heat. Throughout the series, they have been 3 ways red Swords have been achieved. These ways include using Nezuko Kamado's Blood Demon Art, raising the temperature of the blade with a strong grip, and clashing two Nichirin Swords together (the last two ways can only be achieved when the Demon Slayer Mark is present).

Trivia

 * Their namesake contains the kanji for "sun", "ring", which could translate to either "sun ring" or its most common meaning, "solar." In addition to the kanji for "sword/blade", the full translation to the Nichirin Sword is "solar blade."
 * The ore used to forge the swords are likely based off of Hihi'irokane, which literally means (Brilliant Scarlet Metal), a legendary metal of Japanese myth that shines brightly in a manner that it resembles flames or the sun. The metal is considered to be similar to other mythical ores such as Orichalcum of Greek and Roman myth and as such is said to have had magical properties.