The Nightmare Great World is a peculiar place. Although it, like other worlds, has a boundary barrier, this barrier is as unique as the world itself. It's strong, but not excessively so, and sometimes even changes shape to lure powerful beings passing by.
To put it bluntly, if not for the prevention of the Void Storms, the Nightmare Great World wouldn't even need a boundary barrier, because the Nightmare creatures do not fear invasions from other worlds; in fact, it is the countless worlds in this Star Domain that have suffered from Nightmare invasions.
Therefore, when Nether River Ancestor and other powerful beings attempted to breach this world, they didn't spend much effort to break the boundary barrier and invade.
