Chaos

Chaos 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, Chaos 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 Demiurge, one of the twelve Dark Lords. Demiurge was deposed following the Great Multiversal War. His death (and subsequent imprisonment) has shattered the once more or less stable realm, resulting in Chaos as seen today.

Geography

Chaos is a fully flat realm composed of little more than elemental chaos. A void that's alternatively burning, getting filled with water, being torn apart by violent (and three-dimensional) hurricanes or being temporarily stabilized into something resembling small and uneven planets (rarely bigger than a few kilometers, though). And that's without mentioning the times when it freezes, is filled with sounds that are loud enough to explode organic matter, or something even more unique. In short - its very existence is anathema to posthumans and all other life forms (save for elementals), though divine creatures are capable of stabilizing bits of it temporarily.
Nobody knows 'why' it remains two-dimensional. There is a surface that you can walk on, even when there is nothing there to support you. Incorporeals can freely fly above and beneath the line, so elementals only somewhat 'observe' the line. It's useful for arrivals from outside, at least if someone's crazy enough to try it. At least some crazier theories suggest that the immaterial surface might be either the Demiurge's accidental creation (during his death throes), or even his body.
Before the Great Multiversal War, this realm was significantly more sane. The elements cooperated with each other, resulting in a flat world of great beauty - and volatility. Significant part of casualties during the human invasion on Chaos was due to constant floods, hurricanes, tornadoes, irregularities of sound, sudden temperature drops and rises (to deadly levels). It was at least partially due to the Demiurge's micromanagement of his personal universe. When Mankind damaged him to the point where he lost control over them, the entire realm came apart in its seems.