Navira

Navira is one of the Throne Worlds - universes existing on a radically higher energy level than the much more common Vassal Worlds. As a result of this, Navira simply doesn't play according to rules recognizable to inhabitants of more 'natural' planes of existence. In harsher words, it would be nonsensical - if it didn't follow its own rules.
In the far past, it was the homeworld - and seat of power - of Susanoo, one of the twelve Dark Lords. He was deposed following the Great Multiversal War, however his influence still permeats the entire plane, with every single detail of this universe once sculpted by his insane (and only probably existing) hands.

Geography

Navira is technically not an endless plain. However given that its actual size is estimated to be about 300 billion kilometres in diameter, calling it 'endless' does have some basis - after all, if you would need to travel for several lifetimes to reach the end of it, it's de facto endless for your species. It is in fact resembling the concept of an Alderson Disk - a megaengineering habitat the size of a star system.
There is a star in the center of it, its light somehow spreading equally throughout the entirety of the disk. This allows for some relatively pleasant environment, although one of eternal (although not very warm) summer. There is a vast selection of lifeforms who form an environment, including some trees (rather rare sight though, it's mostly the plains), grass, and a number of animals. The ecosystem tends to be on the simpler end of scale, though.
Nobody knows what exactly is on the verges of Navira - the Dominion has only confirmed the size of it, but never released any footage. of how the edges (or the 'underside' of the world) look like. There is also the fact that the Dominion never liked the idea of people digging too deep in Navira (although this problem is mostly eradicated by the ragestorms' and current lack of advanced techmaturgy.

Localized Phenomena

The most common phenomena - that also shapes the Navira as it is known today - are the ragestorms. They are storm fronts of anomalous weather, during which blood rains from the skies and all lifeforms caught under it go into a homicidal, berserkerish rage (while manmade materials tend to quickly evaporate). Even the animals are influenced (although most of them learned to avoid the storms).
The real problem is that plants are somewhat influenced too. Centuries of eating them (or animals that ate them) has had a profound influence on the inhabitants of Navira, turning them increasingly violent over time. Being caught in the ragestorm itself is still fatal, so locals are mostly either nomads or live underground in the rare caves of this land.
The true nature of the ragestorms is that they remnants of the local Dark Lord, whose true form was that of a million kilometres-wide stormfront. He was split by Mankind into so many pieces that it could no longer maintain its own existence (resulting in his soul leaving the body and being trapped by the System). Mankind has also altered the Reality somewhat, making the resulting ragestorm partially repel themselves due to a weird form of winds they generate when in close proximity, to avoid said storms from merging.