In the world of Demon Slayer (Kimetsu no Yaiba), the strongest sword is known as the "Nichirin Blade." The Nichirin Blades are special swords used by the Demon Slayers to combat and slay demons.

Each Nichirin Blade possesses unique properties and abilities, including heightened durability and the ability to harm demons. The color of the blade corresponds to the Demon Slayer Corps member's breathing technique and manifests differently for each individual.

Among the Nichirin Blades, the strongest one is considered to be the "Sun Breath" (Hi no Kokyū) Nichirin Blade. This blade is associated with the Sun Breathing technique, one of the most powerful and advanced breathing styles within the Demon Slayer Corps. The Sun Breathing technique is characterized by its intense flames and aggressive combat style.

The Sun Breath Nichirin Blade, wielded by the protagonist Tanjiro Kamado, is known for its exceptional strength, versatility, and the ability to emit powerful and concentrated flames. It is highly regarded for its destructive capabilities and its effectiveness in combating demons.

It's worth mentioning that the strength of a sword or a Demon Slayer's combat abilities also relies on the skill, training, and determination of the wielder. The Nichirin Blade, including the Sun Breath variant, serves as a powerful tool for Demon Slayers, but its true strength lies in the skillful and dedicated individuals who wield them.