What is the origin of Zhenwu Emperor?

Zhenwu Emperor is the Taoist protector in Chinese folk beliefs, also known as "Zhenwu Laozu", "Taishang Laojun" and "Beidou Emperor". Legend has it that it was created by Zhang Daoling, the second founder of Taoism. Because of his complete merits, he was named Zhenwu Emperor and became one of the Taoist gods. Zhenwu Emperor is regarded as one of the representative gods in Taoist culture, which mainly represents the merits of governing the world, exorcising demons and protecting believers. His image is usually a statue with blue face and fangs, a sword in both hands, a dragon's foot, a dragon robe and a Taoist robe. He is one of the most majestic and mysterious gods in Taoism.