The Echo Realms
Of all the scholarly topics concerning the Multiverse, by far the most contentious and debated is the nature, and even existence, of what are called the Echo Realms in the Standard Multiversal Model. These planes are traditionally defined as the extraplanar realms that are reflections, distortions, or echoes of another plane, and thus do not have a unique existence of their own.
The Standard Model
Historically, this concept was first developed by the Drankorian scholar Theophesus, who collected some of the first maps of the Feywild and proposed a physical correspondence: a mountainous region in the Material Plane is likely to be echoed by mountains, or something like mountains, in the Feywild. In later writing, he linked the Dreamworld and the Plane of Souls in a similar way, considering the Dreamworld to be a metaphysical echo, analogous to the physical echo of the Feywild.
Some time later, Gaius Devarro, one of the most prominent early metaphysical cosmologists in the Faculty of Metaphysics, proposed to unify the old Drankorian concept of demiplanes with the Echo Realms. He proposed one of the first unified theories of the Multiverse, which grew into the Standard Multiversal Model. In this model, the fundamental duality of the Material Plane and Plane of Souls still echo with the powerful magic of the Riving, which separated the Energy Realms and the Plane of Magic from the Material Plane. This echoing energy both shaped the permanent Echo Realms, and still resonates, allowing powerful magic users to shape it into pockets of extraplanar space, although these usually cannot persist for more than a minute or so. Thus, in Gaius Devarro’s model, as adopted by the Standard Multiversal Model, there are numerous Echo Realms, some of which are fully co-extant with their source plane (as the Feywild is with the Material Plane), while others are only partially co-extant (as, for example, the domains of the Shadowfolds are thought to be), or temporary (as the minor extraplanar pockets created by magic).
Criticisms
The conception of the Echo Realms in the Standard Multiversal Model, however, has been extensively criticized. One of the most prominent criticisms originally came from Emyr of Tafolwern, who vigorously argued that the conception of an Echo Realm requires the designation of a primary, or source, plane and a secondary, or reflected plane, and yet there was no evidence at all to support the idea that, for example, the Material Plane should be considered primary to the Feywild. Indeed, he argued that the entire concept of Echo Realms was itself fatally flawed. Instead, we should imagine the existence of four primary realms at the center of the Multiverse, representing the physical world (the Material Plane), the magical world (the Feywild), the spirit world (the Plane of Souls), and the imaginary world (the Plane of Dreams). These four realms, he argued, are the only four planes that can be considered complete: they link the magical and the spiritual, and they extend to fill the entire cosmological space they occupy. Other so-called Echo Realms, such as the domains of the Shadowfolds or the demiplanes created by magical effects, are limited in time, space, or both. Variants of this criticism, often differing only slightly in details, are frequently published, most recently in the works of the dwarven scholar Ulfgar Frostbeard, who specifically focused on the Feywild as a realm of magic manifest, just as the Plane of Souls is a realm of creation manifest and the Material Plane is a realm of energy manifest.
A related criticism was raised by the Stoneborn metaphysicist Yendalo, who postulated a multiversal model that divides the planes into the Inner Realms and the Outer Realms, based on their position with respect to the Divine Veil (more commonly known as the Land of the Dead). To Yendalo and his followers, the division of the Inner Realms beyond the split between the Energy Realms and the other Inner Realms was arbitrary, as it is not obvious what property the Feywild and the Dreamworld share that is not also shared with the Plane of Souls and even the Material Plane.
The fey themselves rarely engage in metaphysical cosmology, but the few scholars among them simply laugh at the claims of the Standard Multiversal Model.