The Black Tortoise is one of the Four Symbols of the Chinese constellations. It is sometimes called the Black Warrior of the North, and is known as Xuanwu in Chinese, Genbu in Japanese, Hyeonmu in Korean and Huyền Vũ in Vietnamese. It represents the north and the winter season. Although its name in Chinese, Xuánwǔ, is often translated as Black Tortoise in English, it is usually depicted as both a tortoise and a snake, specifically with the snake coiling around the tortoise.
In ancient China, the tortoise and the snake were thought to be spiritual creatures symbolising longevity. During the Han Dynasty, people often wore jade pendants that were in the shape of tortoises. Because of ancient Chinese influence on Japan, honorific titles and badges in Japan often referred to the tortoise or images of tortoises.
In the classic novel, Journey to the West, Xuánwǔ was a king of the north who had two generals serving under him, a "Tortoise General" and a "Snake General." This king had a temple at Wudang Mountains in Hubei, thus there is a "Tortoise Mountain" and a "Snake Mountain" on the opposite sides of a river in Wuhan, the capital of Hubei.
In Taoist legend it was said that Xuánwǔ was the prince of a Chinese Emperor. However, he was not interested in taking the throne, but decided to study in Tao's way. At age 16, he left his parents to search for enlightenment in Tao's way. It was said that he eventually achieved god status and was worshipped as a god of northern sky.
Other Chinese legends also speak of how the "Tortoise General" and a "Snake General" came to be. During Xuánwǔ's study to achieve enlightenment and god status he was told that in order to fully achieve god status, he must purge all humanly flesh from his body. Since he was born he had been eating the food of the world, humanly food, therefore his stomach and intestines were still human. Legend told of an event that a god came and changed his human stomach and intestines for a godly body so he could fully achieve god status. (It was also said that the stomach and intestines that were tossed out became the "Tortoise Mountain" and "Snake Mountain".) The stomach and intestines taken out by the god who did the surgery on Xuánwǔ were said to have taken on the shape of a tortoise (stomach) and a snake (intestines). As many Chinese legends speak of certain animals becoming demons over time as they gain knowledge, that's what the tortoise and snake became, and terrorized people. As Xuánwǔ, now in his god status, heard of this, he came and slayed the demons from his past. However, he did not kill them, as the snake and tortoise demons showed remorse. He let them train under him and atone for their wrong doings, and they became the "Tortoise General" and "Snake General", and they assisted Xuánwǔ with his quests.
Once Xuánwǔ's had begun study of the way, he discovered that he must purge himself of all his past sins to become a god. He learned to achieve this by washing his stomach and bowels (intestines) in the river. In the washing of his internal organs, his sins melted from them and into the river in a dark, black form. These then formed into a black tortoise and snake who terrorized the people. Once Xuánwǔ learned of this, he returned to conquer the forms of this past sins and subdue them under himself and they became his servants.
North gates of Chinese palaces were often named Xuanmu Gate after the mythological creature. The place Li Shimin set to kill his brothers Li Jiancheng and Li Yuanji is the gate named Xuanwumen, i.e. the north gate of the Tang palace.
