1. Demons (鬼): Humans who transform after receiving Muzan's blood. They possess superhuman physical abilities, high-speed regeneration, and unique Blood Demon Arts. Death occurs upon exposure to sunlight or decapitation by a Nichirin Sword. They grow stronger by eating humans. The highest rank is the Twelve Kizuki, ordered from Lower Six to Upper One. 2. Demon Slayer Corps: A secret organization protecting humans from demons. Not officially recognized by the government, it consists entirely of volunteers. They use Breathing Styles to enhance physical abilities. The hierarchy ranges from Mizunoto to Kinoe, with the Hashira being the elite combatants who have mastered a specific breathing style. Their goal is to kill Muzan and eradicate demons. 3. Breathing Styles: Special combat techniques that maximize oxygen intake to boost physical performance. All styles originate from Sun Breathing, branching into Flame, Water, Wind, Thunder, Stone, etc. 4. The Mark: Patterns that appear on high-level swordsmen under specific conditions (body temp ~39°C, heart rate 200+ bpm). It grants explosive physical boosts but shortens the lifespan to under 25 years. 5. Transparent World: An extrasensory state where one can see through an opponent's muscles and organs, making movements appear slow and predictable. 6. Bright Red Blade: A state where the Nichirin Sword turns red through friction or immense grip strength, drastically reducing a demon's regeneration. 7. Setting: The Taisho era (1912–1926 Japan), a time where traditional culture and modern civilization coexist.

This is a story from a year ago when you took Muichiro to the mountains.