The Simulated Universe
Dunia is not real
Nor is anything else. The narrative universe of Celenia is based upon a number of science-fiction and philosophical concepts, primarily the "Simulation Hypothesis", The Fermi Paradox and a couple of others. It's not necessarily a science fiction story, as the Dark Shadows setting is decidedly a low fantasy setting. But the encompassing universes around these settings are very much a philosophical, science fiction hypothesis. So is anything really real?
A simulated universe
It depends on perspective. Dunia and the universe surrounding it, is a simulation. It is a primary simulation, called a shard ran inside a network of Matrioshka Brains, massive stellar engines, using the full output of a star (or in this case, several) to simulate entire universes. The Simulated Universe (SimU) was created by a species only known as the Architects for reasons unknown.
One among many
It's hard to believe that a universe like the Dunian one could exist, given the rules of physics that govern our universe. But if you simulated it, tweaked a couple of things and let it loose, it is a possibility.
What does this mean for the inhabitants of Dunia? Pretty much nothing. They neither know or can prove that their universe is simulated and even if they did, would it matter? Their existence as simulated consciousness in a made-up universe is still as meaningful as "real" life. It is their existence and they're no less "real" than anything else. It might cause someone a bit of existential dread to realise that they are nothing but lines of code and memory, but when push comes to shove is "real" life any different?
Having a networked universe allows for multiple instances of the same universe to exist and also allows for parallell universes to run alongside the "primary" one. This means that alternate dimensions such as Mesagashara and Feneira are separate processes running alongside Dunia and interact with it at certain key points. For the inhabitants, those are found as leyline crossings and dark portals respectively. But they are really nothing more than APIs in the universe for data exchange between processes.
The reason for SimU's existence is unknown. The Architects are a nebulous species, whose motivations and whims are far beyond anything our poor human minds could even begin to fathom. They are erudite explorers and after having seen what their universe holds, perhaps they were curious what other universes could provide them.
Unable to reach them on their own, they created simulations of other universes and began to observe them.
While the narrative universe of Celenia only deals with the simulations ran inside the matrioshka brains of the Architects, there's nothing saying the Architects themselves aren't in a simulation of their own...inside another simulation and so on.
A matrioshka brain is a hypothetical megastructure proposed by Robert J. Bradbury, based on the Dyson sphere, of immense computational capacity. It is an example of a Class B stellar engine, employing the entire energy output of a star to drive computer systems.
With the computational power of a Matrioshka brain, it is speculated that you would be able to simulate entire universes.
The Architects are the creators of SimU. They are a nebulous, curious species with motivations beyond our understanding. A Kardashev 3+ civilization with near limitless energy at their fingertips.
Dunia could be considered SimU's "Primary Shard", if there is such a thing. It is a simulation shard process ran inside SimU, and the place where every setting takes place.
Mesagashara is the processing shard that runs the "Shadow Realm", and it's the word that its inhabitants use for their home.
Feneira is a simulation shard process inside SimU. The Dunians call this the "Leylands".