Shadow Gate

The Shadow Gates are permanent portals that lead to the Plane of Shadows, Since the Plane of Shadows (to some extent) mirrors the Material Plane but also allows vastly faster travel, the Imperium constructed a network of Shadow Gates between the major cities of the empire to allow goods to flow at a vastly higher speed. Note that teleportation would be even faster (e.g. using an Arcana Projector) but that is prohibitively expensive.

On the Plane of Shadows the Imperium constructed a set of guarded outposts to guide travelers between the various Shadow Gates. Since the fall of the Imperium these outposts have all been abandoned or destroyed.

When the demonic horde invaded the Imperial City in A.I. 400 the demon lords sent the bulk of their army into the Shadow Gates in order to carry out surprise attacks on the major cities of Aegeron and prevent the inhabitants of the Old Continent from mounting a defense against the demonic invasion. It was due to the heroic sacrifices of the soldiers of the Imperial Legion manning the outposts on the Plane of Shadows, who managed to destroy most of the gates, trapping themselves and the demons in the Shadows.