Demon

Note: "Demon" in White Snake is actually a rough translation of Chinese word "妖 (Yao)".

妖 (Yao) or 妖怪 (Yao-Guai), in a narrow sense, is a category of supernatural beings in Chinese folktales. It generally means "monster with magical powers". Although it was roughly translated into "demon" in White Snake, its meaning is very different from the hell-bound beings in popular culture.

Becoming a spirit
Simply speaking, Chinese folktales believes that an animal or a plant, sometimes even a lifeless object, will become self-aware under certain circumstances, such as when it has lived for a very long time or received other mythical beings' help. Consequently, this enlightened animal (or plant) will no longer be considered as a "natural being" but as a Yao.

This is the first stage of a Yao’s life, sometimes is also called "Becoming a spirit" or "Becoming a monster". At this point, a Yao is basically an intelligent unnatural animal (or a plant). And it also might take few years to develop the ability to speak in human language.

Cultivation and becoming a human
Once a Yao is self-ware, it will begin to cultivate some sort of magical energy inside its body which allow it to perform some supernatural abilities and greatly extend its lifespan. This process is called "Cultivation". The most common ways of cultivation are meditation and practicing mystic arts. But Chinese folktales also believe that a Yao can also try to devour the life force of human or other living things, even another Yao, to accelerate this process.

During its cultivation, the magical energy inside Yao will slowly change its body and appearance, which usually makes it more and more like a human. Eventually, after hundreds of years, it will gain a complete human form. And this is the second stage of a Yao’s life which is also called "Becoming a human". However, this doesn't imply that a Yao will turn into a real human once it has gained enough magic energy. By contrast, It becomes a shapeshifter with supernatural abilities that can transform between its human form and original form at will.

In folktales, a Yao, at this stage, normally prefers its human form which allows it to directly interact with human. Some tales believe that there are also ways to force a Yao to turn back into its original form such as magical traps or poisons. On the other hand, if a Yao loses or consumes too much magic energy inside its body, it will lose its magic abilities, including the ability to shapeshift, and also turn back into its original form, sometimes even faces the risk of death.

Also common in many folklores is Yao's reliance on magical energy to sustain their human appearance as well as halting (or even reversing) the process of aging. This energy must come from a source, usually another living being or another Yao. Hence many Yao's are depicted as evil predators in folklore.

Transcendence and becoming an immortal
There is also a third and very rare stage of a Yao’s life called “transcendence” or "becoming an Immortal", if it continues its cultivation after gaining the human form. If it has achieved this goal, it will transcend into Heaven and become a truly immortal being, also known as a "Xian" ("Immortal").

The roles of Yao in folktales
In Chinese folktales, Yao are normally described as eccentric outsiders or potential dangers to the human world to say the least. They are much more powerful than ordinary people and, unfortunately, have very different moral senses, or even don’t have any moral senses at all, due to their origins and life experiences. In many stories, even a kind-hearted Yao would cause disaster or harm other people without hesitation, if he/she thinks it’s necessary. Hence, the relationship, especially love relationship, between a Yao and a human is at least highly controversial, if not totally forbidden.

On the other hand, even with powerful magic, Yao still can be deceived and betrayed. And human sometimes can be more monstrous and dangerous than Yao. In those stories, Yao usually became the victims of human’s greed.

As a result, though Yao are often imprisoned or slayed by heroes or Taoists or Buddhist monks at the end of stories, their roles in folktales are very different from each other. In some stories, they are villains who harm people or bring disaster to human society; in some stories, they are eccentric hermits who like testing people and appreciate brave heroes; in other stories, especially love stories, they can also be heroes and heroines who bravely challenge the rigid social rules and injustices.

In most folktales, Yao and human are mortal enemies. Many Yaos transformed from their animal form and likely experienced the dangers of being hunted by humans for food in their youth. Once transformed, the advantage turns to their favor and their chance to exact vengeance. Humans view Yaos with great fear, as most encounters with Yao result in human fatality. Relationships with Yaos are considered foolish as Yaos will eventually reveal their animal instinct to kill their prey - humans.