Demon

Demons are an ancient race of monstrous, supernatural, and immortal beings and one of the main antagonists of the series.

Characteristics
Demons come in a variety of forms. Most look human at first, only with slight changes, such as paler skin, catlike eyes with unusually-colored sclera, dark markings that come in various patterns across their bodies, and/or a change in hair color. The changes differ from Demon to Demon, as some demons can still retain their normal appearance while others gain monstrous features like multiple heads, mouths, or limbs.

Demons are a mostly carnivorous, vampiric species whose primary diet is humans, consuming their flesh and/or blood. Sunlight is deadly to them and will burn them to ash, so they strictly operate at night. Demons have an abnormally powerful physical constitution and regenerative ability which makes them immune to normal weaponry and grants them the ability to regenerate from most wounds with ease. Only the Nichirin Blades, special weapons made from special ore with sunlight absorption properties, allowing then to inflict life-threatening wounds on demon kind. Of course, even with a Nichirin Blade, the only method guaranteed to kill a Demon is by chopping their head off. They are also known to hate Wisteria flowers 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. Demons however do still retain their original personality, except it often turns into a twisted, dark version of what it used to be. 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 and Kokushibo. All demons do regain their memories moments before they die.

The Demon Slayers are the central adversary to the demon race as 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 in fact shown to be highly territorial and fight over both prey and potential hunting grounds, the sole exception is the Spider Demon Family, a group of demons forced to work together by their leader Rui in what was a hostile environment. Nonetheless, demons are all for the most part connected to their progenitor, Muzan Kibutsuji, who through an array of curses is able to read their minds, witness what they see and discern their location, and even destroy them remotely, very much operating like a hive-mind of sorts. All demons pledge obedience to Muzan who relies on fear and manipulation to obtain their loyalty. However, Muzan's control over his progeny is not omnipotent as an individual with a strong will can in fact sever their bond with Muzan rendering them immune to his curses and undetectable by him. The only beings known to do this are Tamayo and Nezuko.

The demon species is also known to mutate and grow stronger over time from the more humans they consume, allowing them to grow into more monstrous forms and develop Blood Demon Arts, an array of signature supernatural powers which are unique to each demon. Different human blood types have different levels of nutrition for demons, the more rare the blood type, the greater the nutrition. The rarest blood type is known as Marechi; eating a human with this blood type provides a demon with roughly the same nutrition as eating one hundred ordinary humans.

Both Tamayo and Nezuko are also shown to be exceptions to this rule, the former is shown to have become a very powerful demon despite surgically modifying her body to require minimum blood consumption (typically in the form of blood donations). The latter, who has never consumed any ounce of human life, gathers most of her strength through extensive sleep and was able to develop her own powers and grew stronger over time.

Muzan's own blood is also shown to have special properties that can strengthen a demon, proportional to the amount of blood infused. The blood, however, is dangerously toxic in higher concentrations and can force a mutation in the recipient's body far beyond what they can withstand, causing death, but, if they do acclimate through the process which is shown to be excruciatingly painful for high doses, their strength will grow tremendously. Conversely, Muzan is also shown to be able to nullify a demon's immortality and regenerative ability, being able to devour and kill them if he so desires.

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 belief 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 "Blue Spider Lily" medicine of the doctor, whom he had killed. The Blue Spider Lily however could not be found. 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 "Blue Spider Lily". This makes Muzan the progenitor of all demons.

Creation
Demons are not born naturally, excepting Muzan, the very first demon who was born as a result of the Blue Spider Lily medicine. Most members of the race reproduce by converting a human through exposure to the blood of a very strong demon; only Muzan has been shown to possess this ability. People resistant to demon blood exist, however they are very rare.

Tamayo was also able to convert a human into a demon, although through science rather than the sharing of blood, however this method is nowhere near as efficient as it took her two hundred years to develop, and was put into practice only once. Yushiro was also able to demonize Chachamaru through an unknown way.

It should be noted that their creation is not a mystical, but a biological process achieved through substances, suggesting Muzan's blood and the Blue Spider Lily medicine have mutagenic properties. As evidenced by Muzan pouring blood into a civilian, the process is performed at a cellular level, where if the mutation is too drastic, the victim's cells unable to withstand the change will be destroyed leaving only ash behind.

Restoration
Although not known at large, a demon's transformation is not permanent and can in fact be reversed. A drug that can reverse demonification was developed by Tamayo after hundreds of years of research using the blood of many strong demons that have high concentration of Muzan Kibtsuji's blood for testing. The drug's effectiveness seems to be dependent on the target, as a demon as powerful as Muzan was able to resist its effects and remain demonified although it is known to have taken a massive biological effort to do so.

Current 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 demons upper echelon was destroyed, culminating with their king, Muzan was successfully eliminated during a struggle until dawn which ended with the demon progenitor burned to ashes. Although Muzan would attempt a final push for survival by possessing the Demon Slayer, Tanjiro, and turn him into Demonhood, this attempt resulted in failure and in Muzan's final defeat and by proxy drove the Demon race to near extinction, leaving Yushiro and Chachamaru the race's only survivors. 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.

During the modern era, Aoba Hashibira, a botanist, discovers surviving specimens of the Blue Spider Lily but due to carelessness, causes all of it to wither. With seemingly all the flowers gone, the demon race has no apparent hope of resurrection.

Abilities
Accelerated Growth: Demons grow at rates superior to any human, usually dependent on the number of humans they have consumed and the amount of Muzan Kibutsuji's blood in their body. This allows even the weakest human beings, like Rui when he was a human, to become of the most powerful beings in existence in months. This allows most demons to reach further heights that is impossible to attain when human as long as they keep assimilating Muzan's blood. 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.

Disease & Poison Immunity: Demons have an innate immunity to all types of diseases and poisonous substances, with complex anti-demon drugs and the poison of a Wisteria flower being the only exception.

Enhanced Senses: Demons have innately keen senses, being able to detect blood and poison accurately, from particularly far distances as well. They can also sense the aura of everyone in the vicinity, detecting their presences, allowing them to discern demons from humans, or even perceive the strength of their enemies.

Immortality:  Demons possess eternal youth and can live for centuries, sometimes even longer if they're incredibly strong. They are unable to die but can be killed by sunlight, through decapitation by Nichirin Blades, wisteria poison or if Muzan wills it.

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 outclass Tanjiro Kamado in terms of speed, moving faster than he can perceive or react. Even the weakest members of the Twelve Kizuki, like Rui, can cause Tanjiro to cough up blood with casual punches and kicks.

Menacing Presence: Demons have a naturally menacing presence and aura that evokes fear, with stronger demons having a more intense aura.

Regeneration: Demons have incredible regenerative abilities far superior than any living being or human due to the properties of Muzan Kibutsuji's demonic blood, able to heal any sort of physical injury in mere seconds, including deep cuts, organs, chunks of flesh, entire limbs or even their head and entire body. Stronger demons can heal more serious injuries faster and more precisely than weaker demons. For example, the average demon would take a dozen seconds to regrow an entire arm, while the likes of the Upper Ranks can repair it faster than even most Demon Slayers can register, making it seem like it happened instantaneously. The fastest and most impressive regeneration feats belong to the Demon King, Muzan Kibutsuji, who can heal any injury so swiftly that it makes it seem to marked Hashira that he never got injured in the first place.

Shapeshifting: All demons all have a shared ability and attribute of shapeshifting through the precise manipulation their body. For weaker demons, shapeshifting is automatic and usually random. For example, humans that recently became demons could have horns, sharp nails, fangs or extra limbs that are granted through demonization. Demons that possess more of Muzan's blood would be able to control their own shapeshifting more precisely, enabling them to change their appearance at will. As the progenitor of the shapeshifting ability shared among all demons, Muzan's shapeshifting is the most powerful, being able to change his aura and appearance at will, fooling even the strongest demons, like the members of the Twelve Kizuki.

Supernatural Blood: All demons have supernatural and special blood that originate entirely from Muzan Kibutsuji. It is the main source of all the abilities, characteristics, attributes and capabilities, thus being the nexus and 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, magic that can be used through a demon's blood.

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 which they achieve through their incredible regeneration. demons can also endure pain and discomfort better than humans and even the likes of Hashira, taking on onslaught of attacks as if they were nothing. This could be once again attributed to their innate regeneration.

Weaknesses
Sunlight : The greatest threat to nearly all demons is sunlight as it 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 and Tanjiro.

Nichirin Blades: Nichirin Blades 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: The wisteria flower is poisonous to demons, and releases a scent that can repel demons. Wisteria flowers can be used to create deadly and highly potent poison mixtures to easily kill lower ranked demons, and later contributed to the death of Upper Rank 2 Doma.

Curse of Kibutsuji: Every demon created through Muzan's blood is also inflicted with a deadly curse which activates if they call out Muzan's name or divulge any information about him. Demons like Tamayo and Nezuko, however, have freed themselves from the curse through a special method.

List of Known Demons
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Status as of Anime
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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're depicted in traditional tales.
 * They are both immortals 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 for Demons).

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