The Realm of Shadow

The Realm of Shadow, aka the Shadow Plane, is a plane overlaid across the Prime Material Plane. Because distance is distorted on the Shadow Plane as compared to the Prime, it is possible to travel great distances very quickly by using Shadow portals.

In Aegis
While trying to flee the siege of Malvont, the party was teleported to the Plane of Shadow. In Shadow Ravinia they found a portal to Sigil with an inscription in Old Common signed by Talos.

Cain decided the Shadow Plane was a more vampire-friendly location than the Prime, what with its abundant and delicious negative energy, and walked off into the shadows here.