Just as every civilization with the capability for interdimensional warfare does, the Fallen Apostles have set up extensive defensive systems in every border world they control. Apart from interferer stations and disruptors lingering in the Void, these systems also include "passage locking" facilities that are integrated with the World Barriers.
We all know how the process of "jumping into" a world from the Void goes. If a world is seen as a database wrapped in a specific encoding method, then a Void Fleet is an additional set of data with a special encoding way and responsive frequency. This additional data must be tuned and translated, modifying its "frequency" to approach the world and changing its "encoding" for mutual interpretation with the world before it can enter a Material Universe.
