Oblivion is a fictional realm inhabited by Daedra in The Elder Scrolls series of games. While Oblivion is often inaccurately described as "The Elder Scrolls version of Hell," the metaphor expressed in the game actually implies a complex supernatural cosmology. According to the lore of the The Elder Scrolls universe, Oblivion is the void surrounding Nirn, the mortal plane of existence. The planets, nebulae and other astral bodies in the sky are the mortals' view of the spheres of the Daedra floating in that void. Stars are holes on the inner surface of Oblivion through which the energy of Aetherius shines into the mortal realm. This is also true of the sun, which is known as Magnus. The exception is the constellation of the Serpent, Sithis, which is made up of unstars (through which, presumably, the energy of Oblivion shines into Aetherius).
Each of the Daedra has a different personality and sphere of influence, much like the pantheon of gods in a polytheistic religion. Each Daedra has his or her own plane of Oblivion. Oblivion refers to all of the dimensions of which the daedra occupy, which is contrary to the popular misconception that Oblivion refers only to the realm of Mehrunes Dagon. Daedra act according to their qualities as personified forces of nature and shape their region of Oblivion to fit them. Some mortals will worship a given Daedra which exemplifies traits they consider to be virtues.
An afterlife or the mortal belief in one is sometimes referred to in The Elder Scrolls lore, but to date its exact nature has never been confirmed. Mortals do not "go to Oblivion" after death. For their part, Daedra are considered immortal, but if sufficiently weakened a Daedroth (singular) may be remanded to Oblivion, eventually to return to its own sphere.
The barrier between Nirn, the mortal realm, and Oblivion, have been protected in the past, and prevented from opening due to a series of barriers which, with each game, have been weakening; the death of the emperor and removal of the Amulet of Kings finally allows the barriers to be breached broadly. The main plot of The Elder Scrolls IV: Oblivion involves the death of the Emperor, the subsequent opening of the gates, and the disastrous consequences as aggressive Daedra invade the mortal realm. At the end of The Elder Scrolls IV: Oblivion the player closes the gates to Oblivion forever. This, however, is contradicted by the expansion pack The Elder Scrolls IV: Shivering Isles when the player can enter a gate into a different realm of Oblivion to do quests for a Daedra lord.
The character Mankar Camoran states in game that Tamriel, the main continent in Nirn, was once part of the oblivion realm of Mehrunes Dagon, and that the invasion of Tamriel is in fact a "liberation". However, whether this is fact or propaganda is unknown. Wikipedia
