The realms of Oblivion: their plurality, the correspondence between a realm and its ruler's sphere, and the constraints governing passage between planes.
LESSON 01
Plurality and Correspondence
Establish the basic structure.
Many realms, not one place
Oblivion is a collection of distinct realms, each belonging to a Daedric Prince, rather than a single undifferentiated region. The realms differ radically in landscape, inhabitants and governing conditions, and a student who imports a unified underworld model from elsewhere will misread in-world accounts immediately. Plurality is the first and most consequential fact about Oblivion.
Each realm expresses the nature of its ruler. Coldharbour, associated with Molag Bal and domination, and the Deadlands, associated with Mehrunes Dagon and destructive upheaval, differ from Apocrypha, associated with Hermaeus Mora and the accumulation of knowledge. The correspondence is close enough to be predictive: knowing a Prince's sphere tells a scholar a great deal about what their realm will be like.
A small number of episodes grant direct access to Daedric realms, and these carry disproportionate evidentiary weight because they replace in-world report with depiction. Coldharbour during the Planemeld, the Deadlands during the Oblivion Crisis, Apocrypha in connection with the Solstheim episode, and the Shivering Isles associated with Sheogorath are the principal cases.
Consequently our knowledge of Oblivion is highly uneven: a few realms are described from within in considerable detail while most are known only by name and reputation. Students must resist generalizing from the well-documented realms to Oblivion as a whole, since the sample is not random but consists precisely of those realms that intruded on Tamriel.
Passage between Mundus and Oblivion is possible but constrained, and the constraint is a maintained condition rather than an absolute barrier. The Oblivion Crisis demonstrated that the protection depended on specific dependencies which, once removed, allowed gates to open at scale. Smaller summonings and localized incursions across many periods confirm that the boundary is permeable under the right conditions.
Setting the Planemeld beside the Oblivion Crisis isolates what is general from what is particular. The former attempted to merge Nirn into a realm; the latter opened breaches for invasion. Different Princes, different mechanisms, different defences. What they share is dependence on mortal collaborators and on infrastructure that could be located and attacked, which is why mortal resistance mattered in both.
Most descriptions of realms not directly entered come from summoners, cultists, captives and hostile theologians, each with strong reasons to describe what they saw as they do. A cult's account of its patron's realm and an imperial cult's account of the same place will differ systematically, and neither is disinterested.
Firm conclusions are available: realms are plural, they correspond to spheres, passage is possible and constrained, and the Princes are not a unified bloc. What cannot be concluded from present evidence is the internal nature of realms never entered, their number with precision, or the mechanism by which correspondence between Prince and realm obtains.