Demon

Demons (鬼) are a race of monstrous, immortal and malevolent beings and one of the major antagonists of the series.

Most 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.

Characteristics
Demons come in a variety of forms. Most look human at first, only with slight changes like paler skin, catlike eyes with unusually-colored sclera, dark markings that come in various patterns across their bodies, 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 blood and with an strong aversion to sunlight which burns 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 can regenerate most wounds with ease. Only the Nichirin Blades, special weapons that have been exposed to sunlight can inflict life-threatening wounds, with beheading being the preferred method to kill them. They are also known to hate Wisteria flowers which are poisonous them and whose smell is repudiated by them.

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 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. 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 but gathers most of her strength through extensive sleep and who was able to develop her own powers and grow 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, 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, so they may help him finding it. 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 and his top two Twelve Kizuki, Doma and Kokushibo, have 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 for testing. The drug's effectiveness seems to be dependant 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 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 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.

During the modern era, the descendant of Inosuke Hashibira, Aoba works as botanist discovers the remnant of the Blue Spider Lily but due to carelessness, causing all of its to wither. With all the flowers are gone, the demon races have no hope of resurrection.

Abilities

 * Immortality: Demons possess eternal youth and can live for centuries, sometimes even longer if they're incredibly strong. They can't die through conventional means, but can be killed by sunlight or with one of the Demon Slayer Corps' special Nichirin Blades.
 * Enhanced Regeneration: Demons have a strong regenerative ability more than any living being or human, able to heal any sort of injury caused by a conventional mean in mere seconds. That includes regrowing limbs and organs.
 * Inhuman Physical Capabilities: Demons possess inhuman strength, speed and endurance.
 * Blood Demon Arts :  These are spells that Demons can cast due to their special blood. The look and abilities of these spells can vary, for example, Rui can manipulate fine, sharp threads and Daki can turn into and manipulate special sashes.
 * Growing Power: The strength of a specific Demon depends almost entirely on how many humans they have eaten, and they can also grow stronger by receiving additional blood from Muzan, which is how the Twelve Kizuki gained their overwhelming power. 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.

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 Muzan: 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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