The ley-network is a vast web of arcane energy created by Vash at the height of his power that runs "underneath" the fabric of Creation. It serves as a massive barrier to outside influences from other planes and dimensions, although it is by no means impregnable. However, weak beings attempting to enter Creation are often ensnared or destroyed outright by the network's coursing energy. Intelligent or powerful entities may sneak through the ley-network or force themselves through (oftentimes with considerable damage).
Nital's influence was stemmed for a time by the ley-network, until the Blood God himself was freed from his prison and tore a massive hole in its fabric. Sorna's influence from Multor was also limited until the aforementioned event, and now malevolent beings flow freely through the damaged areas. Benevolent gods, as well as the Chromatai, repair portions of the ley-network, but it is an ongoing process. The ley-network is so vast that portions of it will always be damaged or in the process of being repaired. The Obscuring Mists provide another layer of hindrance for those attempting to penetrate it.