Nexus:(1) Pre-severing, the sum total of imaginings contained within the Astral/Ethereal Conjunction. (2) Post-severing, the subset of imaginings immediately surrounding the origin. (3) The astral/ethereal prison fabricated by the Severing.
- --Astinus the Chronicler
The Nexus is a mysterious location likely somewhere linked to each Axial City (e.g. Sigil) that has thus far only be accessible through the Four Doors or Train 88. According to Astinus, the Nexus was once a much larger, more unified location, until an event known as "the Severing" (see below). Currently, the Nexus is split into an "outer", slightly more accessible set of locales (see Outer Nexus below), and an "inner", completely inaccessible set of locales (see Inner Nexus below).
The "physical" location of the Nexus is unknown. Despite the collective grouping of the locations in both the Outer Nexus and Inner Nexus, it is likely that each location is in itself a separate demiplane. Regardless, all seem to be located at least partially, within the Astral, the Ethereal, or some combination of both. As a result, Nexus, like Sigil, appears to lie within the Astral/Ethereal Conjunction.
The severing was the physical and metaphyscial separation of the area around the Tower (i.e. Strahd's Castle) from the remaining Nexus demi-planes. It has been hypothesized that this event was accomplished by Eliath in order to both isolate and imprison anyone within (e.g. Sougad Lawshredder).
Below are highlighted the various means through which portions of the Nexus have been accessed.
At least between 30 HR and 33 HR, Train 88 was the primary method to reach the Nexus from Sigil. This access was likely restricted to the Dabus and/or the Lady. Once in the Nexus, Train 88 has likely been the primary mode of transportation between locations or Nexus demiplanes, at least prior to the severing.
In Episode: Barovia Hills Cop, the Azrai traveled on Train 88 from the demiplane of Barovia to Raak-Po. Moreover, during the same episode, the Azrai found a broken track that once lead to Strahd's Castle (i.e. physical evidence of the severing).
The Four Doors have led to various portions of the Nexus.
Eschaton SHARD technology has forced its way into the Nexus on more then one occasion.
In Episode: Eliath's Gambit, the Eschaton were using energy from the Sigil power grid to open gates for Train 88 from the UnderSigil Palo Alto Station to the Nexus cold mountain fortress of Dratsacurra.
The Outer Nexus includes all of the demiplanes depicted on the Protean Map excluding the areas immediately surrounding Strahd's Castle (see Inner Nexus below).