There are many stories about the Underworld gate most of which even he didn’t know which one is true and which one is fake.
What he does know about the Underworld Gate was that during the early Tang dynasty they once fought for the domination of the world by supporting other people with the Righteous Temple supporting Li Shimin
Before Tang dynasty was established, the Underworld Gate was the leading heterodox sect as they are very connected with the Sui dynasty officials and many officials of Sui dynasty at that time were said to be members of the Underworld Gate. Whether that is the truth nor not, nobody knows the truth
But Bai Qingniu was convinced that Wu Hou, Wu Zetian had connection with the Underworld gate.
He even believes that Wu Zetian herself was a member of this mysterious s
