Like most of the secret areas in the game, there is one in chapter 4 called the Purple Cloud Mountain. To get to this area, you need to defeat the Venom Daoist twice. After defeating the boss, he will unlock the new secret area that has several adventures and the Scorpionlord Boss fight. Here is how to get to the Chapter 4 Secret Area: Purple Cloud Mountain in Black Myth Wukong.
How to Get to Chapter 4 Secret Area: Purple Cloud Mountain in Black Myth Wukong
Venom Daoist Locations 1
Go to The Webbed Hollow > Webbed Hollow > Pool of Shattered Jade Shrine. Head straight into the cave and you will find some cocoons blocking the path. Beat them or simply pass through them quickly. Follow the cave path and at the end, you will find a large cocoon hanging from a tree. Start hitting the Old Cocoon and at mid-health, the Venom Daoist fight will begin.
Venom Daoist Locations 2
Go to The Webbed Hollow > Temple of Yellow Flower > Court of Illumination Shrine. Head into the Court of Illumination and go through the guards straight to the other end. Keep following the path and take a left when you come across a blueish stone. Use the above images to find the right path.
Go down and you will find the boss there. Once you have defeated the Venom Daoist the second time, he will unlock the Purple Cloud Mountain.