Demons are entities created after a Dark Spirit possesses a being of the physical realm; destroying their soul in the process and altering their physical form entirely. Demons are malicious existences, creatures of nightmares who only exist to spread chaos without reason. They are the representation of entropy, betraying one another on instinct. Demons exist only for themselves, obtaining nourishment from the suffering and anguish of those surrounding them. Travelers consider demons extremely dangerous, more threatening than their spiritual form. This terrifying belief stems from the power they obtain after a perfected transformation. Demons are spiritual beings who are able to interact with both realms without hinderance. The physical confine they obtain after destroying the soul acts as a physical anchor, while retaining the spiritual prowess they possessed originally. The mutation they undergo grants demons an unimaginable increase in physical might.
In the universe, there are two forces which dictate fate. These forces are contradictory; opposite yet interconnected. Represented by a spirit of light and a beast of darkness, yin and yang, push and pull, good and evil, these two energies cycle until the ending of time. They will only vanish temporarily should one perish; undergoing rebirth through their opposition. This rule dictates both the spiritual realm and physical world. When aspects of peace overpower aspects of entropy, spirits and creatures live in tranquility and serenity while experiencing chaos during the victorious ages of entropy. However, when compared to the creatures of the physical realm, spirits are extremely sensitive to the alterations of peace and chaos. The terms "good spirits" and "evil spirits" do not exist. Just as humans, there is darkness and light in all creatures of the spiritual realm. However, when there is an abnormality in the universe or evilness outweighs righteousness, spirits are prone to undergoing changes according to the influences of their connected areas. They become Dark Spirits. Creatures of negativity and darkness. These dark spirits are the representation of chaos and feed off the suffering, anguish and malicious energies of both the physical and spiritual realm.
Dark Spirits are considered incredibly dangerous due to possessing supernatural abilities and an entropic instinct. But it was after the Hundred Year War that humanity experienced the true threat of a realm overran by spirits of darkness. When a spirit bonds with a creature of the human realm, they imbue the host with an increase in both physical and mental prowess; while the mixture of essences causes the creature to undergo physical transformations that matches the spirit. Unfortunately, the process kills the host. In history, there is only one case where a spirit successfully bonded without deteriorating the human in which they resided in. And the unification created a link between the human and spirit realm. After closing the bridge, human, animal and spirit unification was forgotten. Where humans once feared traversing the spirit wilds, men gained the ability to ignorantly journey through deep forest.
Thousands of years passed since the first Avatar chose to close the link between the human and spirit realm. Unfortunately, he had not foresaw the outcome of removing humanities ability to understand the connection and importance in regard to one another. There were plenty instances where spirits of darkness would appear in areas of negativity and chaos. One popular example are the Kemurikage, spirits who haunted tyrannical fire lords. Fortunately, the eras of chaos was rather short lived compared to the years of peace and tranquility; filtering the darkness and ushering an era of peace. As day creates night and night creates day, the age of peace transforms into years of chaos. But the negativity, suffering, anguish, pain, and unresolved emotions that plagued humanity during the Hundred Year War brought an even deeper evil.
Leagues of spirits was influenced by this influx of negative energy; creating an era of chaos in the spiritual realm. The sudden unbalance served as a secondary bridge that allowed the spirits to transfer into the physical realm where suffering occurred. From there on, these malicious beings followed a dark instinct. When Dark Spirits possess living creatures, the concentration of negative energy destroys the host's soul. Simultaneously their essence pours into the creature's body, contorting and twisting it. Transforming the host into an entirely new existence, Demons. Demons are the result of a Dark Spirit possessing a physical confine which is devoid of its soul. Their essence gradually mutates the physical body into a form more suitable to its spiritual existence. It is the process that living creatures undergo during possession by tranquil spirits. As seen in the time of Spirit Wilds, spiritual essence overwhelms the souls of living creatures; influencing them physically by giving them certain characteristics. Only the negativity of dark spirits act as a catalyst for the overall transformation and soul destruction.
The experience and time undergoing a perfected transformation varies throughout the host. Creatures of a weakened will undergo a swift rebirth while beings of stronger power can take an eternity; criteria for the process depending on multiple variables. All creatures of the physical realm and certain inanimate objects are susceptible to this horrific result of dark spirit possession. Initially, animals were the prime targets for dark spirits due to their connection to nature yet weakened mental and spiritual energy. Allowing the soul-destruction process to occur immediately after possession, creating demonic beast meant to destroy everything in its process. However, as the Avatar died and the spiritual realm spiraled into chaos after a century of war, dark spirits gained an exponential thirst. Soon they started to seek stronger creatures, humans. The process for a human to become a demon ranges from an immediate transformation to a prolonged mutation. The result of reaching perfection is the destruction of one's soul, completely resulting to primal instinct.
Due to inhabiting the physical realm, demons trade their spiritual traits with an unimaginable increase in physical might. They also possess special abilities connected to their overall existence and representations.