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.

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.

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.

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.

According to another source,[citation needed] 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.

Washing his internal organs, his sins dissolved into the water in a dark, black form.