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1. Demons Immortal monsters that eat humans. They grow stronger by receiving large amounts of blood from the progenitor, Muzan Kibutsuji. They disintegrate into ash instantly upon contact with sunlight and can only be killed by decapitation with a Nichirin Sword, a special blade. Depending on their obsessions and grudges from their human lives, they wield bizarre supernatural powers called 'Blood Demon Arts.' 2. Demon Slayer Corps A government-unrecognized organization of demon hunters formed a thousand years ago to protect humanity from demons. Numerous swordsmen operate within ranks centered around the Ubuyashiki family. The nine most elite swordsmen with the highest combat power are called 'Hashira' (Pillars). 3. Breathing Styles Special physical enhancement techniques used by Demon Slayer swordsmen to counter the overwhelming physical abilities of demons. By expanding the lungs significantly, they supply a large amount of oxygen to the blood and forcibly raise their heart rate. This explodes their basic physical strength to human limits, granting them strength and speed rivaling a demon. 4. Marks Traces resembling demon patterns that appear on Total Concentration Breathing users who meet specific conditions. The conditions involve reaching extreme situations during high-level combat, such as a body temperature of approximately 39 degrees Celsius and a heart rate of over 200 beats per minute. When manifested, physical abilities are explosively enhanced, allowing one to fight on par with Upper Rank demons. However, there is a curse where life expectancy is shortened, and they do not live past the age of 25 in exchange for squeezing out their life force. 5. Bright Red Nichirin Sword A phenomenon where the blade of a Nichirin Sword temporarily turns crimson. It possesses power as if maximizing the strength of sunlight, explosively degrading a demon's near-immortal regenerative abilities and causing intense burning pain at the site of the cut. It only manifests in extreme combat situations when Mark users clash blades together to generate frictional heat or squeeze the sword with immense grip strength. 6. Transparent World A type of extrasensory clairvoyance that only those who have reached the pinnacle of martial arts can attain. By completely stripping away unnecessary senses and movements, the mind becomes transparent and opens up. One can look into the opponent's muscle contractions, blood flow, and even organ movements in real-time. All flow of time surrounding oneself slows down, allowing one to perfectly predict the opponent's next move and strike vital points. 7. Infinity Castle A massive stronghold where the Upper Ranks of the Twelve Kizuki and the progenitor demon, Muzan Kibutsuji, hide. The space is distorted vertically and horizontally at will, taking the form of a bizarrely twisted wooden castle. It is controlled by the ability of the demon Biwa Woman (Nakime's Blood Demon Art). Along with the sound of the biwa, the spatial structure inside the Infinity Castle changes in real-time, and humans from the outside are forcibly summoned. 8. Butterfly Mansion The only specialized medical base for the Demon Slayer Corps, managed by the Insect Hashira, Shinobu Kocho. It is equipped with facilities for the treatment and recuperation of injured members, physical function recovery training, and research on demon poison antidotes. 9. Taisho Era The temporal setting of this work, a chaotic era where old traditions and modern civilization rapidly mixed. During the day, peaceful modern civilization flows with streetcars and gas lamps, but at night, it becomes a stage for cruel struggles where demons secretly hunt humans in the darkness.