Good brothers tell fortune.

The specific process is as follows: There are two people in Chang 'an, one is a woodcutter and the other is a fisherman. They are two good brothers. One day, they sang poems. One says the river is good, and the other says the mountain is good. The woodcutter asked the fisherman why it was good. The fisherman says he can catch big fish every day. Why? Because there is a fortune teller in Chang 'an named Yuan Shoucheng, who is Yuan Tiangang's uncle. I can always tell fishermen where there are big fish.

This was heard by a water fork in Jinghe River and told the Dragon King. The Dragon King was furious, because fishermen caught fish and were the "aquarium" of the Dragon King of Jinghe River, or, in layman's terms, his people. So the Dragon King became a man and asked Yuan Shoucheng to calculate the weather tomorrow. Yuan Shoucheng truthfully said that the Dragon King didn't believe him, so he made a bet. The Dragon King calls himself the Rain Dragon God and doesn't take Yuan Shoucheng seriously. In this way, the next day, the Jade Emperor told the Dragon King that it was going to rain, and the time and place were exactly the same as Yuan Shoucheng said. The dragon king was frightened. Because he has promised Yuan Shoucheng, if he doesn't promise, he will be driven away. However, in order to save his face and protect the Jinghe aquarium from fishermen, the Dragon King changed the time and count without authorization and broke the dogma.

The Dragon King turned to Yuan Shoucheng for help, saying that Yuan Shoucheng had drunk the Dragon King and publicly told him that he was the King of Jinghe. The dragon king was frightened. Yuan Shoucheng said that the Dragon King had committed a dogma and would be beheaded. The Dragon King begged Yuan Shoucheng to save his life, so Yuan Shoucheng led him to intercede with Emperor Taizong today, because Wei Zhi, who was driven away by Emperor Taizong, was the official in charge of beheading the Dragon King. If he interceded with Emperor Taizong, he might be all right. Thus, it leads to the story of Emperor Taizong's going to the underworld and his later journey to the West.

As for why Wei Zhi is the official in charge of executing the death penalty, perhaps Wei Zhi is a god in The Journey to the West. . .