The Amethyst Bamboo is the core of the defenses that the human Spiritual Gods laid down to protect the Subsidiary Realms that humanity escaped to. The Primal Chaos Spiritual God's family, called the Primal Family, are one of the ones that have inherited a copy of the Amethyst Bamboo as a guardian object. Because of this, the demon beast clan’s Spiritual Gods can not force their way into the Dreamcloud World.
At one point Nie Li says that the Amethyst Bamboo is a treasured herb that is rarely seen between the heaven and earth. They’re usually buried under the earth, continuously absorbing the essence between the two. Some Amethyst Bamboo popped out from the ground after being nurtured for several hundreds of years. Therefore, when it began to grow, it’ll only grow in few centimeters in a year. After spending a few thousands, or even tens of thousands of years in the earth, the Amethyst Bamboo will germinate and push its shoots above ground. Once it does so, it can grow up to several meters in height in just a single day and will continue to grow rapidly.
Note: Nie Li later states that some of what he said was an exaggeration. It is unclear if he was referring to the facts about the Amethyst Bamboo or not.