The Shadowfell/ Plane of Shadow

colorless echo of the Material Plane

recognizable landmarks, twisted and warped

shadow crossings: shadows and dark places may conceal gaps in the Empyrean Weave, allowing passage to the Shadowfell

the thick melancholy of the plane tends to seep into those there for an extended period of time.

holds backdoors into the Dark Domains: the Dark powers of the Shadowfell created or nurture the prisons of darklords, carrying little for bystanders in their capture.
something about the plane seeks to ensnare creatures of significant magical power. Dragons, for example, can easily step through a portal to the Shadowfell, but require aid in order to leave, otherwise finding themselves trapped there.

Planar backdoor to The Raven Queen, Matron of Death 's domain

Divine can better influence the echo planes than the Material