Demon

Demons are an ancient race of monstrous and malevolent beings and are the central antagonists of Demon Slayer: Kimetsu no Yaiba.

Overview
Demons are a carnivorous, vampire-like species whose primary diet is humans. They are also nocturnal as they are weak to sunlight, though they also stay up during the day as they don't require sleep. Demons have an abnormally powerful physical constitution and potent regenerative abilities, making them immune to normal weaponry and granting them the ability to heal from any wound with ease, even re-growing their head. Only the Nichirin Swords, weapons made from special ore with sunlight absorption properties, can inflict life-threatening wounds. However, even with a Nichirin Sword, the only method guaranteed to kill a demon is through decapitation. Demons are also known to hate wisteria, which are poisonous to them and whose smell is repudiated by them.

Practically all demons possess a murderous instinct to kill and devour humans, and have lost most of their memories from their human life. While they do still retain their original personality, most times it becomes a darker version of what it used to be, often corrupted by their core beliefs and their past. However, there are a few demons who have completely retained their human emotions and memories, such as Nezuko Kamado, Tamayo and Yushiro. Certain members of the Twelve Kizuki also retain their human memories such as Rui (though he eventually lost his memories later on), Kaigaku, Kyogai, Doma, and Kokushibo. All demons do regain their memories moments before they die, which possibly means that demons also have their lives flash before their eyes before they turn into complete ash.

Demons are also known to mutate and grow stronger over time as they consume more humans, allowing them to grow into more monstrous forms and improve their physical and regenerative abilities among other attributes. Once demons consume enough humans, they can develop Blood Demon Arts, an array of supernatural powers and techniques which are unique to each demon. Different human blood types have different levels of nutrition for demons: the rarer the blood type, the greater the nutrition. The rarest blood type is known as Marechi; eating a single human with this blood type provides a demon with roughly the same nutrition as eating one hundred ordinary humans.

The Demon Slayers are the central adversary to the demon race. They are an organization trained to destroy them through a multitude of well refined techniques and tools developed across centuries to exterminate them. As a result, demons tend to live in secrecy with the population at large being ignorant to their existence. Even more, some demons blend into human society as a way to hide from persecutors and also to have a much easier access to prey.

Demons for the most part work in solitude not bothering to group up and are instead shown to be highly territorial and fight over both prey and potential hunting grounds. A notable exception is the Spider Family, a group of demons forced to work together by their leader Rui, with permission to do so granted by Muzan himself.

Demons are all, for the most part, connected to their progenitor, Muzan Kibutsuji All demons pledge obedience to Muzan, who uses fear and manipulation to maintain loyalty. However, Muzan's control over his progeny is not omnipotent. An individual with a strong will might be able to sever their connection with Muzan, rendering them immune to his curse and making them mostly undetectable by him.

Humanoid
These demons are identical to normal humans at a glance, and mostly resemble their appearance when they were human with only with slight, sometimes unnoticeable, alterations to their physiology, including but not limited to: paler skin, pointed nails, strange pupils and unnaturally colored sclera (that may or may not be due to their demonification).

Inhuman
These demons have humanoid bodies, but with prominent alterations to some aspect of their physiology that make them appear inhuman, unnerving, or noticeably alarming. The majority of lesser demons fall in this category, sharing common physical traits including but not limited to: curved horns, prominent fangs, discolored veins around the forehead, elf-like ears, and specialized markings that resemble birthmarks or tattoos that appear somewhere on their bodies.

Mutated
These demons are heavily mutated and barely resemble humans. Their appearance are akin to beasts, yōkai, and traditional oni. These demons can have mutations that range from multiple appendages, disfigured bodies, and morphed faces. Some demons are more monstrous and animalistic, while some are more abstract and eldritch.

Creation
Demons are not born naturally; they are all originally humans who have been transformed into demons later in life. The very first demon, Muzan, underwent an inadvertent transformation when his doctor administered him an experimental medicine containing the primary component of the Blue Spider Lily, in an effort to cure his terminal illness. After becoming a demon, he discovered his vulnerability to sunlight, which caused severe burns upon exposure. Throughout his thousand-year existence, Muzan sought to create more demons, driven by the hope that one of them would undergo a mutation that would grant them the ability to withstand sunlight. This pursuit was not aimed at increasing the overall number of demons, but rather focused on finding the Chosen Demon whom he can absorb to grant him the power to overcome his sole weakness - sunlight.

As the progenitor of demons and the source from which nearly all demons originate, Muzan is the only person in the world who possesses blood that has the unique power to transform humans into demons. His cells permeate all the demons he creates, serving as the foundation of their existence. Consequently, when Muzan is eliminated, all the demons he has spawned will cease to exist.

New demons are created by exposing humans to Muzan's blood, which triggers a transformative process, provided the individuals can withstand the resulting mutations. It should be noted that this is not a mystical process, but a biological one performed at a cellular level, suggesting Muzan's blood and the Blue Spider Lily medicine have mutagenic properties. There are three primary methods by which demonification is accomplished:
 * 1) Forceful injection by Muzan: This method involves Muzan personally injecting a controlled amount of his blood into a human, achieved through attacks or by taking advantage of an injured person. The contact of the victim's open wound with his blood initiates the transformation. Muzan's attacks can be swift and inconspicuous, such as when he swiftly slashed the neck of a passerby in a bustling Asakusa street without anyone even noticing. Notable examples of successful transformations through this method include Nezuko Kamado, Asakusa Demon, Akaza and Enmu.
 * 2) Recruitment by Muzan: This method involves Muzan seeking out vulnerable humans he deems worthy to become a demon and convince them to willingly accept his invitation to become a demon. He then shares his blood with them, which triggers the transformation. Notable examples of successful recruits through this method include Tamayo, Kokushibo, Hantengu and Rui.
 * 3) Recruitment by an Upper Rank: This method involves the Upper Ranks of the Twelve Kizuki providing their blood to humans they deem strong enough to become a powerful demon. The blood of these Upper Ranks alone does not possess the power to convert humans into demons. Instead, it serves as a signal to Muzan that they intend to transform the recruited individual. Once Muzan grants his approval, the blood undergoes a transformation, becoming capable of turning humans into demons. This invitation can only be extended by the Upper Ranks. The recruited individual who receive the blood should be immensely grateful and are strictly forbidden from spilling even a single drop of the blood. Consequently, the Upper Ranks must find individuals who willingly accept the invitation. Notable examples of successful recruits through this method include Daki and Gyutaro, who were recruited by Doma, and Kaigaku, who was recruited by Kokushibo. Over the course of his many centuries of existence, Akaza attempted to recruit several Hashira he encountered in battle, including Kyojuro Rengoku and Giyu Tomioka, but each one rejected his offer without exception.

Due to the alteration of bodies by Muzan's blood, having a greater quantity of his blood results in increased power. However, the majority of humans are unable to withstand the extreme transformation caused by a large amount of his blood, leading to the destruction of their cells and their bodies disintegrating into ash. Consequently, it is challenging for Muzan to find individuals who possess the resilience to withstand a sufficiently high concentration of his blood necessary to transform into an Upper Rank-level demon.

Those who practice Breathing Styles exhibit greater resilience to the transformative effects of Muzan's blood. As a result, a significantly larger quantity of Muzan's blood is required and a longer period of time is needed for Breathing users to undergo transformation into demons. For instance, the process of transformation for Kokushibo required an extensive period of three full days to complete. This stands in stark contrast to the relatively quick transformations observed in other demons, such as the Asakusa Demon, which occurred in mere seconds, or Nezuko, whose transformation took several hours. Following this, Muzan lost interest in turning swordsmen who employ Breathing techniques. Additionally, there are individuals who possess complete resistance to his blood, although they are exceptionally rare.

In addition to Muzan, Tamayo is the only other person who is able to create demons, albeit through scientific means rather than blood-sharing. However, this method is far less efficient, as it took her two hundred years to develop and was only put into practice once when she converted Yushiro to save him from a terminal illness. Prior to the final battle, Tamayo also succeeded in transforming her cat, Chachamaru, into a demon.

Restoration
Although not known at large, a demon's transformation is not permanent and can in fact be reversed. A drug that can reverse the transformation into a demon was developed by Tamayo after hundreds of years of research using the blood of many strong demons that have high concentration of Muzan Kibutsuji's blood for testing. The drug's effectiveness seems to depend on the target, as a demon as powerful as Muzan was able to resist the effects, although it took massive biological effort to do so. Nezuko Kamado is one of the most prominent examples of a demon who has been successfully turned back to human.

Origin
The first demon that is said to have existed was Muzan Kibutsuji. The one who turned him into a demon was a generous doctor from the Heian Period, who wanted to save Muzan from death since, at the time, he was diagnosed with a disease which would kill him before he turned twenty. Out of frustration and thinking that the treatment would be useless and only cause him more suffering, Muzan killed the doctor before the latter could finish the treatment by letting Muzan consume the final part of his Blue Spider Lily medicine.

Shortly after, Muzan came to realize that the treatment had in fact worked and the effect was just delayed, prolonging his life, although at the cost of his humanity. He soon regretted killing the doctor, as he found out that becoming a demon made him unable to expose himself to sunlight anymore, as it burned him, and the only way to overcome the sunlight was by consuming the medicine made by the doctor, whom he had killed. Obsessed with walking under the sun once more and getting rid of his weakness, Muzan has made his the life goal to find the Blue Spider Lily by creating demons with his blood, in hopes that one day, a demon would come along to either be able to conquer the sun or be able to find the rare flower.

Status
Following an arduous operation carried out by the Demon Slayer Corps in which they found themselves trapped in the Infinity Castle, the vast majority of the demon population was destroyed. Muzan was also successfully eliminated during a struggle until dawn which ended with the Demon King being to ash under the sunlight. Although Muzan would attempt a final push for survival by turning the Demon Slayer Tanjiro Kamado into a demon, this ultimately failed. With Muzan's defeat, all demons created through his blood have died, leaving Yushiro and Chachamaru as the only demons to have survived. With no demon bodies to preserve as well as recording historical documents of demons to be made, the knowledge of the demon race eventually fades into obscurity and mythology.

During the Reiwa era, a botanist named Aoba Hashibira discovered surviving specimens of the Blue Spider Lily, but due to carelessness, all specimens withered and were lost. With all the flowers gone, the demon race has no apparent hope of resurrection. The only demons who are still alive is Yushiro and Chachamaru.

Accelerated Growth Rate
Demons grow at tremendously rapid rates, dependent on the number of humans they have consumed and the amount of Muzan Kibutsuji's blood they have assimilated. This allows even the weakest humans, like Rui, once a frail and weak young boy, to become one of the most powerful beings in existence in mere months. As a result, demons are able to reach further heights that are impossible to attain when human as long as they keep assimilating Muzan's blood and consuming humans. Some demons, however, have a limit to how much they can use to grow stronger, and they might be not compatible with a higher amount of Muzan's blood and die if they don't consume enough humans.

Biological Absorption
Some demons have displayed the ability to absorb biological matter into their bodies through physical contact, mainly as an alternative method for consumption. It is also implied that demons can kill other demons by absorbing them into their bodies, as Kokushibo absorbed the demons that lost to him in Blood Battles. Muzan also used this ability to nullify a Blood Demon Art that pierced his body.

Enhanced Senses
Due to their predatory nature, demons possess very sharp sensory abilities. Most notably, demons can discern normal blood from marechi blood. Even a low-level demon like the Slasher was able to trace the smell of a bento box back to its maker from a great distance.

Information Sharing
Demons possess the ability to disseminate and transmit information to one another through psychic means. Muzan had used this information sharing ability to show an image of Tanjiro Kamado to Enmu after giving him orders to kill him. Gyokko also stated that he recognized Muichiro Tokito's Demon Slayer Mark from the information of Tanjiro he received from Muzan. Doma was privy to the poison that Shinobu Kocho used against the Daughter Spider Demon on Mount Natagumo. Muzan was also able to give Kokushibo orders during the Infinity Castle Arc. Kokushibo also knew of Tanjiro's existence and the fact that his brother's Sun Breathing still remains through this ability. Some demons like Akaza could share their thoughts and sights with demons lower ranked than him. Akaza had complained to Muzan that Doma was abusing this ability to constantly bother him after the latter became Upper Rank Two.

Inhuman Physical Capabilities
Even the weakest of demons possess inhuman strength, speed and endurance, being able to outmatch the average person in all physical categories effortlessly. For example, the Temple Demon and Swamp Demon can both outmatch Tanjiro in terms of speed, moving faster than he can perceive or react, in spite of his persistence and skill in combat. Even the weakest members of the Twelve Kizuki, like Rui, can cause Tanjiro to cough up blood with casual punches and kicks, while Lower Rank demons with no combat background, namely Enmu, can evade Tanjiro's attacks and techniques without much effort. A demon's physical abilities are implied to be directly tied to the amount of Muzan's blood they have assimilated and how much humans they have consumed.

Unlimited Stamina & Endurance
All demons have a shared physical trait of endless and limitless stamina and vitality, simply being unable to accumulate fatigue or get tired no matter how hard they try to exert themselves. Demons can also endure pain and discomfort much better than humans and even the likes of Hashira, taking on onslaught of attacks as if they were nothing, such that a severed arm or slashed neck wouldn't even bother them. It is also revealed that demons don't need to sleep. Giyu Tomioka stated that demons don't feel pain or exhaustion.

Menacing Aura
Demons have a naturally menacing presence and aura that evokes fear, with stronger demons having a more intense aura. The aura of Lower Rank demons like Rui can make the air feel heavy and thick with a mere gaze, while Upper Rank demons like Zohakuten and Kokushibo possess auras that can induce fear and physical pain into their opponents or even make them lose the will to fight.

Immortality
Demons do not age, possessing eternal life that allow them to life for centuries without showing the slightest sign of deterioration or aging. They are unable to be killed by conventional means, like suffering excessive injuries, but can be killed by sunlight, through decapitation by Nichirin Swords, Wisteria poison or if Muzan wills it.

Disease Immunity
Demons have an innate immunity to all types of diseases and ailments, with complex anti-demon drugs and poison derived from Wisteria being the only exception, the former being able to effectively weaken demons and the latter able to kill demons if they receive even milligrams of the substance.

Supernatural Blood
All demons have supernatural and special blood that originate entirely from Muzan. It is the main source of all their abilities, characteristics, attributes and capabilities, thus being the most important aspect of demons. The higher the concentration of Muzan's blood a demon possesses, the overall stronger they are. Once demons grow strong enough, they develop supernatural abilities unique to themselves known as Blood Demon Arts.

Weaknesses
Sunlight: The greatest threat to demons is sunlight, which ignites their cells, severely limits their regeneration, and fatally burns them to ashes. The only demons so far to have developed resistance to sunlight are Nezuko Kamado and Tanjiro Kamado.

Nichirin Swords: Nichirin Swords are blades made out of Scarlet Crimson Iron Sand and Scarlet Crimson Ore, which absorbed sunlight to such an extent that a decapitation strike with such a weapon can instantly kill a demon. There are exceptions, however, as Muzan, Akaza, and Kokushibo overcame this weakness and survived decapitation by these blades.

Wisteria: Wisteria is a plant that is poisonous to demons, and releases a scent that can repel demons. Extract from the plant can be used to create deadly and highly potent poison mixtures to easily kill lesser demons in small amounts, and later contributed to the death of Upper Rank Two Doma, though he had to ingest over 700 times the lethal dosage.

Curse of Kibutsuji: Every demon created through Muzan's blood is also inflicted with a deadly curse which activates if they call out divulge any information about him, even something as minor as his last name: "Kibutsuji". When activated, their regeneration will cease and the cells belonging to Muzan inside the demon will kill them. It has been shown to work both automatically, such as when one demon was immediately killed upon accidentally saying his name, or by Muzan himself remotely, such as when he killed another demon on his own accord.

List of Known Demons
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Trivia

 * Demons' eyes are continuously damp, so they don't need to blink.
 * Demons share many similarities with vampires as they are depicted in traditional folklore.
 * They are both immortals humanoid beings with inhuman traits, supernatural abilities, who consume humans, and they both are weak to the same three things: sunlight, a special fragrance (garlic for vampires, wisteria for demons) and a certain metal (silver for vampires, Nichirin Swords for demons).
 * Demons are unable to process human food, when they ingest human food, they simply throw it back up.
 * Demons do not sleep and remain awake during the day, but due to the risk of death should sunlight shine on them, they do not move around much in day hours.
 * Nezuko, Tamayo, Yushiro, and later the Asakusa demon and Chachamaru, are the only demons not under the control of Muzan.

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