The Shadowgate is a portal device in the Realm of Liminalis that the Twiceborn use to move between the Afterworld and the Mortal Plane. The current Shadowgate is a dark, broken gate that is specifically able to teleport Twiceborn into the shadows of Tellus.
Historically, the Shadowgate is the portal Navtix, the god of Shadows and Trickery, used when traveling to Mortalis. This is the reason why it can only teleport the Twiceborn into shadows. When the Cataclysm happened, all of the portals used by the gods to travel to Mortalis were shut except for this one. When the gods built the Realm of Liminalis to help facilitate and house the Twiceborn, Natvix moved his portal here so it could be used by the Twiceborn, for he found he could not use it himself.
The Shadowgate can teleport a small number of Twiceborn to any location on Tellus that currently casts a shadow. This could be a large as the Shroudlands themselves or as small as the shadow of an alleyway in the morning light. However, it can only be used a limited number of times per day, as Navtix is not as powerful as he once was.
In addition to these small-scale incursions, once per day Navtix can provide the power to teleport a large number of Twiceborn to the continent to handle whatever is deemed to be a serious threat.
If a Twiceborn dies in Mortalis, their body and soul are teleported back through the Shadowgate into the Realm of Liminalis immediately. This magic allows for the possibility of resurrection by the gods.