In the Han dynasty (206 BCE – 220 CE), worship of Four Directional deities developed.
With the inclusion of the middle, there were five major directions, each associated with a divine being or beings, a season, and a color (with the "middle direction" being associated with the emperor and the color yellow).
These sets of correlations of five, is part of the elaborate philosophical system of Wu Xing, although some of the basics related to directional deities was much older.
The north was associated with a pair of divine beings, the Dark Warrior, a tortoise and snake creature.
Associations with the season of winter, and with the color black are mentioned. (In some schools of thought the color is considered to be a deep shade of blue)
Each of the directions was also associated with one of the wu xing, or five "elements" (sometimes also translated as "phases" or "materials"): that of the north was water.
According to Anthony Christie, the tortoise and snake combination was known as the Black Warrior.
Although the worship of the other directions was an ancient practice, the worship of the north was avoided because the north was considered the dwelling place of a destructive deity of the ocean wind.
However, the worship of the north was practiced, with sacrificial ceremonies to the Black Warrior, by the rulers of the Han dynasty, which claimed to rule with the protection of water and the north.
Although the Black Warrior (Xuan Wu) is generally depicted as a snake entwining around a turtle, sometimes they are viewed as two separable generals.