日輪刀 — literally 'sun-wheel sword' (日輪 = sun, the disc of the sun in classical Japanese; 刀 = single-edged blade) — is the canonical demon-killing weapon in Demon Slayer. The blade is forged from 猩々緋砂鉄 (shōjōhi satetsu, 'crimson-monkey iron sand') and 猩々緋鉱石 (shōjōhi kōseki) mined exclusively from a peak that is permanently bathed in sunlight. The sun-charged metal is the only material that can sever a demon's neck and keep it severed — ordinary steel, fire, even direct sunlight wound demons but a nichirin-tō decapitation is irreversible.

The most famous feature is the colour change. When a newly-forged blade is first gripped by its swordsman, it shifts colour depending on the user's compatible breath style: red for Flame Breathing, blue for Water, yellow for Thunder, pink for Love, and so on. Kyojuro Rengoku's blade is a deep saturated crimson with visible flame-like patterning, sometimes depicted as actually radiating heat in the manga. Black-coloured blades are extraordinarily rare and historically associated with short-lived swordsmen — the colour that 'doesn't take a side' in the breath classification system.

A further wrinkle: a Flame Hashira in full bloodline-bearing form can heat the blade to an additional state called 赫刀 (kakutō, 'red-blazing sword'), where the steel glows actively rather than just being red-coloured. This state is a specific late-arc unlock and treated as a return to the original Sun Breathing power.