The Dark Echo is an Inner Plane which sits parallel to The Prime Material Plane, sometimes known as The Shadowfell or The Plane of Shadow. It is mostly inhabited by undead creatures.
The Dark Echo reflects places on The Material Plane, but it is a realm of eternal night. The reflections are are twisted and warped, dark parodies of their Material Plane counterparts. Where a castle exists on the Material Plane, a ruin might exist in its place in The Dark Echo, covered in poisonous moss.
Travel
Travel to the Dark Echo can happen by accident, as crossings manifest in places of pain and suffering on The Material Plane. A crossing might be a blot of shadow in the corner of a crypt, an open grave, or a twisted tree on an ancient battlefield.
Travel to this plane can also be achieved through various spells, or via The Ethereal Plane.