Five thousand years ago, the Hell Cave Heaven collapsed, and it was then that many forces learned of its existence.
After its collapse, the internal world of Hell Cave Heaven merged into the larger world outside, but it hadn't fully integrated, creating a peculiar form of existence.
After investigating the situation of Hell Cave Heaven, many forces discovered that its existence threatened the entire Cultivation World.
Therefore, led by all the superpowers of the Cultivation World, a defense line was set up on Hell Island beside Hell Cave Heaven, investing a large amount of manpower and resources to establish a defensive formation to prevent the powerful Hell Creatures of Hell Cave Heaven from escaping.
In fact, although Hell Cave Heaven had collapsed, the state of semi-integration still constrained the Hell Creatures within.
