Shadowfell

The Shadowfell is one of the Planes of Existence in the Critical Role universe. It was the home of Thar Amphala until that city was teleported by Vecna to the Material Plane.

In, Vox Machina chased a resurrected Delilah Briarwood into the Shadowfell, following their encounter with her in a ziggurat in Marquet. There they witnessed, from a distance, Vecna's rebirth atop the tower of Entropis.

Description
According to Eskil Ryndarien, the Shadowfell is a plane of negative energy, and elements of it are used to funnel the transition of souls from life to death or back. It exists as a mirror plane, much like the Feywild. There are elements that resemble Exandria, but twisted, shifted."

When Vex'ahlia shot an Oracle Arrow into the siphon beneath Whitestone, she caught a glimpse of the Shadowfell on the other side: