Aetherius is the realm from which Aedra such as the Eight Divines originate. It can be compared to the Void, from which the planes of Oblivion and the Daedra within them exist. The Void, and by extension Oblivion, are therefore considered the dualistic inverse of Aetherius.  Some have even gone as far as to call it the "Un-Void".
It is the plane of pure magicka, and it's the realm where magicka was said to originate. Common belief is that souls ascend to this location upon death. The Imperial Geographic Society have this to say about it:
"It is aetherial energy that infuses our daily existence, from highest to lowest, and gives all the races of men, mer, and beast common purpose. Its magic brings the rain to the fields, love to our hearths, and scientific principles to our technological industries. It gives us the very Sun itself. Finally, Aetherius is the home to the Aedra, those cornerstones of the Mundus whose aspects we see in the temple, in lordship, and the high walk of heroes."
Realms within Aetherius
It is substantially more difficult to reach Aetherius than it is to reach the Void of Oblivion. The only two successful trips there occurred firstly during the Reman Dynasty, and secondly by the "Sun Birds" of Alinor. From the Reman Dynasty, the Royal Imperial "Mannanauts" were obtained, and from the Sun Birds, the Orrery of Firsthold. It is also rumored that the Dwemer may have reached Aetherius in the past, as well. It is also rumored that the stars are direct portals to Aetherius and that they were created when holes were blasted through Aetherius during the creation of Nirn. Also, the agent sent by Uriel Septim to Daggerfall to ensolve the murder of king Lisandus was transported to Aetherius by Nulfaga in Castle Shedungent in order of find the Mantella. And the Dragonborn reached Sovngarde, a realm of Aetherius, to defeat Alduin in the main quest of The Elder Scrolls V: Skyrim.
Scenes that mention AetheriusHero of Daggerfall entered Aetherius' sub-realm Mantellan Crux to extract the Mantella for use against the Underking. The realm exists as a massive labyrinthine puzzle. Sheogorath is encountered here, in his parlor. Although, in later titles, Sheogorath became associated with the realm of Oblivion, Aetherius' converse, he was not included as such in Daggerfall.
In The Elder Scrolls IV: Oblivion, Aetherius is mentioned in passing in the form of a Heaven Stone named Aetherius Stone. It is located northwest of Skingrad and slightly south of Bleak Flats Cave. Activating it Fortifies Magicka 50 pts and adds Resist Magic 20%.
When seeking entry for the first time to the College of Winterhold, the conversation with the watcher of the bridge may turn so that this response may be given: "I want to unravel the mysteries of Aetherius," to which she responds with acknowledgement that it is a noble goal to wish to understand the plane from which all magic comes from.
- The Elder Scrolls II: Daggerfall (First appearance)
- The Elder Scrolls IV: Oblivion (Mentioned only)
- The Elder Scrolls V: Skyrim
- The Elder Scrolls V: Dawnguard (Mentioned only)
- ↑ 1.0 1.1 1.2 1.3 1.4 Pocket Guide to the Empire, Third Edition: Birthsigns
- ↑ The Magic of Aetherius. The Imperial Library.
- ↑ The Monomyth
- ↑ The Light and the Dark
- ↑ Kafadi, Marcus. A Pocket Guide on Places Not Here. The Imperial Library.
- ↑ 'The Old Ways
- ↑ Magic from the Sky
- ↑ Pocket Guide to the Empire, Third Edition: Arena Supermundus
- ↑ Pocket Guide to the Empire, Third Edition: Birthsigns
- ↑ The Talos Mistake
- ↑ Sovngarde, a Reexamination
- ↑ Aedra and Daedra