Echo is believed to be an outer plane and the destination of souls after death. In most pantheons, Shai is thought to reside in Echo, where they judge each soul and determine their fate in the afterlife.
Only souls and psychopomps are said to be able to access Echo - no living being or other extraplanar creature can travel there. As a result, depictions of the plane vary greatly, as none can be quite sure what it looks like. Some cultures and religions do not believe of its existence at all.