Nichirin Sword

Nichirin Swords are special blades used by Demon Slayers made specifically for the purpose of slaying demons.

Description
Nichirin Swords (among other Nichirin bladed weapons) are the main weapons used by Demon Slayers in the Demon Slayer Corps in order to kill demons. They are forged with Scarlet Crimson Iron Sand and Scarlet Ore, both of which are found on high mountains like the Sunlight Mountain, which is perpetually bathed in sunlight all throughout the year on Mount Yoko.

As they constantly absorb sunlight, decapitating demons with Nichirin Swords can permanently kill them. Once their neck-bone is slashed by the weapon, their heads and bodies disintegrate into ash in a similar fashion to when exposed to sunlight. However, Nichirin Swords are less effective when used to slash other parts of a demon's body, making beheading the only viable option for Demon Slayers.

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.

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.

After the Demon Slayer reaches the rank of Hashira, their weapons will be engraved with the words "Destroyer of Demons". This tradition came some time after the creation of the sword that Tanjiro and Kotetsu found inside the Yoriichi Type Zero, which only had the Slay on it and was created for the sole purpose of annihilating demons.

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.

Weapon 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; however, when giving Tanjiro Kamado his new sword, Hotaru Haganezuka remarked that the past wielder of the blade before its refinery must have been a very strong swordsman. Given this statement, it is implied that a stronger skill level can create patterns or slight changes in a Nichirin Sword's coloration.

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 Sword will turn shining bright red. Bright red Nichirin swords hamper the regeneration of demons and can generate extreme heat that can induce immense pain to demons. There are only 3 known methods to turn a Nichirin sword bright red: using Nezuko Kamado's Blood Demon Art, raising the temperature of the blade with a strong grip, and clashing two Nichirin Swords together (the latter two only capable of being 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.
 * "Iron sand/black sand" specifically higher quality "iron sand" is in fact known in real life to heat up when exposed to direct sunlight, capable of causing minor first degree burns if in contact without proper equipment.
 * Only four (five if counting the late Kanae Kocho) of the nine Hashira use standard Nichirin Swords: Kyojuro Rengoku, Giyu Tomioka, Sanemi Shinazugawa and Muichiro Tokito.