Tanjiro‘s Nichirin Blade Changes Color
Nichirin Blades (日にち輪りん刀とう) are unique blades used by the Demon Slayer Corps to slay demons. They’re made of a special mineral that absorbs sunlight constantly, which is one of the Demons’ only serious flaws. As a result, one of the only ways a human can kill a Demon is to slice the neck bone with a Nichirin sword, with the other being to use a weapon coated in wisteria poison.
When Nichirin blades are initially drawn by their owner, they take on a particular hue, which is why they’re also known as “color-changing swords.” However, the wielder must be skilled, or the sword will not change colour.
Nichirin blades are made from red iron sand and crimson ore found on high mountains such as Mount Yoko’s Sun Mountain, which is constantly bathed in sunlight throughout the year.
After clearing final selection, Demon Slayer recruits choose their own ore.
During the forging process, the ore will be used by a swordsman in the Swordsman Village, and the swordsman will personally deliver the weapon to the Demon Slayer.
The hand guard, or tsuba, on each Nichirin blade protects the swordsman from being cut. It’s found at the intersection of the handle and the blade, and it comes in a variety of colours and shapes. Inosuke Hashibira’s Nichirin Blades had hand guards when Kozo Kanamori initially unveiled his newly forged blades, based on his appearances in the series. Following that, he serrated his swords, removed the hand guards, and wrapped the hilt in bandages.
Each Nichirin blade is unique because it changes colour depending on the user. They come in a variety of hues. Zenitsu Agatsuma’s blade is distinctive in that its coloration is a pattern of lightning bolts that runs the length of the blade rather than a solid stripe. There is no explanation for this pattern.
Each hue is claimed to provide the blade specific characteristics.
Because the characteristics of the black Nichirin swords are unknown, they are seen as a bad omen for the demon slayer who will not succeed (because he dies before he can learn much of things on the blade).
Bright Red Nichirin Blades
A Nichirin blade becomes red as the temperature of the blade rises. Bright crimson Nichirin blades slow demon regeneration and can even burn them at the cellular level, preventing wounds from healing. Heat transfer is responsible for the formation of the crimson red Nichirin blades. Red Blades can be gained in three distinct methods throughout the series. Using Nezuko Kamado’s Blood Demon Art, raising the temperature of the blade with a tight grip, and colliding two Nichirin blades together are some of these methods (the last two ways can only be done when the mark of the demon slayer is present).
Their namesake contains the kanji for “sun” (日にち nichi?), “ring” (輪りん rin?), which could translate to either “sun ring” or its most common meaning, “sun”. Along with the kanji for “blade” (刀とう tō?), the full translation of the Nichirin Blade is “blade of the sun”.
The ore used to forge the swords is probably based on Hihi’irokane, which literally means (Shining Scarlet Metal), a legendary metal from Japanese mythology that glows in a way resembling flames or the sun. This metal is considered similar to other mythical minerals such as orichalcum from Greek and Roman myths and as such is believed to have magical properties.