Aetherius, the Immortal Plane, is the realm from which Aedra such as the Nine Divines originate. Aetherius is the source of magic and of all creation in Mundus. It can be construed as the inverse of Oblivion, from which the Daedra originate; conversely, Aetherius is the realm from which the Aedra are connected. Oblivion is considered the dualistic inverse of Aetherius. [OOG 1]
It is the plane of pure magicka, and it's the realm where magicka originates. Common belief is that souls ascend to this location upon death. The Imperial Geographic Society has this to say about it:
Realms within Aetherius
Though it is known that Aetherius contains a large number of planes within itself, only four are specifically known.
- Sovngarde 
- Mantellan Crux - a self-contained prison for the Mantella.
- Sands Behind the Stars
- The Far Shores
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. Some believe that the stars, including Nirn's sun, Magnus, are direct portals to Aetherius that were created when Magnus and the Magna Ge fled back to Aetherius during the creation of Nirn.[OOG 2] Also, the agent sent by Uriel Septim to Daggerfall to ensolve the murder of King Lysandus was transported to Aetherius by Nulfaga in Castle Shedungent in order of find the Mantella. The Dragonborn reached Sovngarde, a realm of Aetherius, to defeat Alduin in 4E 201.
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.
- The Elder Scrolls II: Daggerfall
- The Elder Scrolls IV: Oblivion (Mentioned only)
- The Elder Scrolls V: Skyrim
- The Elder Scrolls V: Dawnguard (Mentioned only)
- It is likely that the name came from the Aetherius Society, a religious cult formed in the 1950s.[source?]
- ↑ 1.0 1.1 1.2 1.3 1.4 Pocket Guide to the Empire, Third Edition: Birthsigns
- ↑ Nu Mantia Intercept Letter 7
- ↑ The Monomyth
- ↑ The Light and the Dark
- ↑ 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