In Japan, it is one of the four guardian spirits that protect Kyoto and it is said that it protect the city on the north. Represented by the Genbu Shrine, which is located to the north of Kyoto Imperial Palace.
In ancient China, the tortoise and the serpent were thought to be spiritual creatures symbolizing longevity. The Fujianese 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. During the Han dynasty, people often wore jade pendants that were in the shape of turtles. Because of ancient Chinese influence on Japan, honorific titles and badges in Japan also often referred to the turtle or tortoise.
In the classic novelJourney to the West, Xuan Wu 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 Xuan Wu 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 Xuan Wu'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[which?] 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, Xuan Wu 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 Xuan Wu with his quests. (Another legend held that the mortal organs were tossed out to become Wuhan's Tortoise and Snake mountains.)
According to another source, once Xuan Wu 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 Xuan Wu learned of this, he returned to subdue them as in the other story.