In the Thuban continuity, the structure of the universe can be considered macroscopically five-dimensional, with living creatures inhabiting a macroscopic four-dimensional 'sheet' (four-dimensional hyperplane) in the overall space. It's enclosed on both sides by two other spaces that are at least hyperplanes1), Order and Chaos, and technically speaking encompasses a near-infinitely thin sliver between them.
The Order hyperplane+'s forces serve as a structural base for the Thuban universe, providing it with a foundation grid to work with. The Chaos hyperplane+'s forces provide the Thuban universe with entropy to create structures in what would otherwise just be a desolate, dead, rigid space. In combination, the two macroscopic neighbours of the Thuban universe are what makes it cosy and habitable.
Another way of viewing this is to picture a bubble, picturing the interior surface of the bubble as the Order hyperplane+ and the turbulences of air outside it as the Chaos hyperplane+, with the exterior surface of the bubble affected by the drag of the turbulences representing the inhabited universe.
Alternatively, consider this visual analogy:
Those analogies are imperfect because the Chaos hyperplane+ is not aligned with the universe in a different way than the Order hyperplane+, it's simply populated with different forces. These are mostly orthogonal to and independent of each other - a breach of either plane through the Thuban universe hyperplane to touch the other would not result in any tragic side-effects (other than the puncture in the topology of spacetime in the Thuban universe, but I digress).
On that note, in Thuban, Iisharsria can be considered a gateway into the Order hyperplane+, i.e. a place where the Thuban universe doesn't exist, where its topology has been broken, and it's following the path of least resistance - it tries to let its forces flow through the Order grid instead. That only works with limited success, since it's simply not permeable for some of them, but enough to give Iisharsria a tangible manifestation in the Thuban universe.
Conversely, Iisharkaea is a gateway into the Chaos hyperplane+, desperately assimilated with similarly limited success.
It's entirely conceivable - and in fact quite probable - that more of such breaches exist, though they'll on average be much further apart than even space-faring civilisations will discover.