The Barrier is what keeps the Void from consuming all life on Scarterra. The Barrier is a wall of wind approximately ten miles into the Near Void. The wall of wind is no more harmful to mortals than a stiff wind but crossing it exposes the mortal to the life draining power of the Deep Void. The Near Void is covered in ice and snow. If someone were so inclined to dig through the ice and snow they would find a layer of solid silver infused with the Nine's collective power sustaining the Void. The Barrier drastically reduces the amount of heat that the Void would otherwise be siphoning off but more importantly it acts as a wood chipper completely pulverizing any Void Demon that tries to cross it. The Barrier is not perfect and holes will appear temporally in it. These holes are invisible to mortals but clearly visible to Void Demons who are constantly looking for a chance to escape. The Barrier has the most holes and gaps at the anniversary of the the Divine Rebellion at the end of the calendar year and the Barrier is strongest in mid-summer. More powerful demons are larger than weaker demons, so they require larger holes in the Barrier to escape. Therefore most of the Void Demons that make it to the material plane to cause trouble are fairly weak compared. demon grunts are fairly commonly encountered while death demons are rarely seen. Demons larger and stronger than death demons have not been seen at all since the Second Unmaking which was essentially caused by the Barrier collapsing entirely. Even after the Barrier was fixed, there were still countless powerful demons in the material plane and it was too late to stop the damage, only to mitigate it.
Oops I didn't make it! Occasionally Void Demons appear in the material plane as amputees. This occurs when a Void Demon tries to pass through a temporarily hole in the Barrier and the hole closes before the Demon crosses entirely.