Black Turtle-Snake

The Black Tortoise is one of the Four Symbols of the Chinese constellations. It is usually depicted as a tortoise intertwined with a snake.
According to Eastern Han philologist Xu Shen, the character wu meant "knight" in the Jianghuai dialect. So the English translation Black ~ Dark ~ Mysterious Warrior is a more faithful translation.
It represents the north, thus it is sometimes called Black Warrior of the North.
While neither character xuan or wu literally means tortoise or snake, tortoises and snakes are known to hibernate during winter, so xuanwu as a whole represents the season of winter.
The image of intertwined tortoise and snake likely symbolizes a state of inner struggle.
In Japan, the characters xuanwu are pronounced as Genbu. It is said to protect Kyoto on the north side, being one of the four guardian spirits that protect the city. It is represented by the Kenkun Shrine, which is located on top of Mount Funaoka in Kyoto.
An important Taoist priest also has Xuanwu as his clergy name. He is sometimes, as in Journey to the West, portrayed in the company of a turtle and a snake.

History

During the Han dynasty, people often wore jade pendants that were in the shape of turtles. Originally, there was a legend that said that turtles could not mate with other turtles, only snakes. This is why the Black Tortoise is depicted with a snake on its back.
The northern gates of Chinese palaces were often named after the Xuanwu. Most famously, the Incident at Xuanwu Gate, where Li Shimin killed his brothers Jiancheng and Yuanji and seized power in a coup, took place at the north gate of the Taiji Palace, in the north of Chang'an.

Legends

In ancient China, the tortoise and the serpent were thought to be spiritual creatures symbolizing longevity. The Min people custom of building turtle-shaped tombs may have had to do with the desire to place the grave under the influence of the Black Tortoise.

Xuanwu

In the classic novel Journey to the West, Xuanwu was a king of the north who had two generals serving under him, a "Tortoise General" and a "Snake General". This god had a temple in the Wudang Mountains of Hubei, and there are now a "Tortoise Mountain" and a "Snake Mountain" on opposite sides of a river near Wuhan, Hubei's capital. Taoist legend has it that Xuanwu was the prince of a Chinese ruler but was not interested in taking the throne, opting instead to leave his parents at age 16 and study Taoism. According to the legend, he eventually achieved divine status and was worshiped as a deity of the northern sky.
Other Chinese legends also speak of how the "Tortoise General" and a "Snake General" came to be. During Xuanwu's study to achieve enlightenment and divine status, he was told that, in order to fully achieve divinity, he must purge all human flesh from his body. Since he had always eaten the food of the world, despite all his efforts, his stomach and intestines were still human. A god then came and changed his organs with divine ones. Once removed, the original stomach and intestines were said to have become a tortoise and a snake, respectively. The tortoise and snake became demons and terrorized people. Now divine, Xuanwu heard of this and returned to slay the monsters he had unleashed on the countryside. However, as the snake and tortoise showed remorse, he did not kill them but instead let them train under him to atone for their wrongdoings. They then became the Tortoise and Snake generals and assisted Xuanwu with his quests.
According to another source, once Xuanwu had begun his study of the Way, he discovered that he must purge himself of all of his past sins to become a god. He learned to achieve this by washing his stomach and intestines in the river. Washing his internal organs, his sins dissolved into the water in a dark, black form. These then formed into a black tortoise and a snake who terrorized the country. Once Xuanwu learned of this, he returned to subdue them as in the other story.

Seven Mansions of the Black Tortoise

As with the other three Symbols, there are seven astrological "Mansions" within the Black Tortoise. The names and determinative stars are:

Mansion no.	Name	Pinyin	Translation	Determinative star
8		Dǒu	Dipper	φ Sgr
9		Niú	Ox	β Cap
10		Nǚ	Girl	ε Aqr
11		Xū	Emptiness	β Aqr
12		Wēi	Rooftop	α Aqr
13		Shì	Encampment	α Peg
14		Bì	Wall	γ Peg

File:Lingshan Islamic Cemetery - turtle tomb - DSCF8473.JPG|thumb|A characteristic "turtle-back tomb" in Quanzhou, Fujian